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I will be on leave so as a precaution I want to see if somebody has entered my room with the door being locked from outside.

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    Put some rat traps near the walls. The intruder(s) are more likely rodent than human.
    – emory
    Nov 7, 2015 at 22:50
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    Is the door the only point of entry? If so, get two or more small blocks and place them at irregular points on the ground behind the door as you close it (assuming it swings into the room) at various points of closure. When you return, open the door slowly and check if they are still in the same position (assuming you remember). It is unlikely that an intruder will notice, let alone know where the blocks were.
    – copper.hat
    Nov 7, 2015 at 23:20
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    There's a neat trick used in Death Note by the main character: set a strip of paper on the opening of the door as suggested and an mechanical pencil lead on the hinge. This way not only you find out if they enter (broken lead), but also if they enter with ill intentions in case they replace the paper. If they have good intentions like hiring a cleaning lady or recover something they borrowed you they'd note it to you or not replace the paper.
    – Francisco Presencia
    Nov 8, 2015 at 2:17
  • @copper.hat "assuming you remember" - take a photo! ;)
    – MrWhite
    Nov 8, 2015 at 14:52
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    Wouldn't you want to also know who it was and what they were doing? Looks like a motion sensitive hidden camera is only about $20
    – Martin Smith
    Nov 8, 2015 at 15:12

12 Answers 12

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I've seen some people close little slips of paper in the doorframe (hinge and/or handle sides) that would fall when the door is opened, and/or stack boxes against the outside of the door in what appears to be somewhat random fashion but where the edges and corners line up with pen or other markings in a particular way, and compare before vs. after pictures.

Edit: The above is the low-tech solution. Logging door sensors and/or a motion activated camera are higher-tech solutions that should not be overlooked by someone who arrives at this question/answer looking for similar ideas.

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    That reminds me of something a man did in the BYU TV show, Granite Flats (which is free to watch, FYI). Anyway, you should be concerned about the person just putting the stuff back (as happened, though somewhat unsuccessfully, in the show). One thing you can do is write a number on each slip of paper and put them in a specific, random order (that you write down and keep). Then, when they scatter, whoever entered the house won't know the order. Make sure they're not visible from the outside before opening the door.
    – Brōtsyorfuzthrāx
    Nov 7, 2015 at 3:05
  • Also, make sure they won't scatter accidentally. You don't want to blame innocent people.
    – Brōtsyorfuzthrāx
    Nov 7, 2015 at 3:11
  • The Blue Peter Intrusion Detection System: use a child's building blocks or some hand-made cardboard cubes with numbers and letters written onto the faces. Arrange some loosely into a little pyramid or whatever behind the door and note the combination. If the room is disturbed so will be the cubes (and the combination).
    – PCARR
    Nov 7, 2015 at 12:39
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    @Shule: "Make sure they're not visible from the outside before opening the door." -- then how are you going to check that they're still there when you return, before opening the door and scattering them yourself? Come in through the window? ;-) Even if you open the door a little bit and stick your arm in with a camera to check your trap, in theory the attacker could do the same thing in order to note the state of the trap before coming in. Of course they'd need to be expecting something, an amateur would blunder through it and then it's too late to replace correctly.
    – Steve Jessop
    Nov 7, 2015 at 12:52
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    I used this method because I suspected my landlady was entering my premises without my permission. It worked like a charm.
    – n00dles
    Nov 11, 2015 at 17:43
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In Dr. No James Bond placed a hair across his door closures which would fall off as soon as somebody opened it.

Of course you have to test if the hair will stay stuck for the time you are on leave. Consider using some glue.

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    That's indeed a pretty good idea and not some Hollywood BS :) While a hair is almost invisible it may not be the right thing if you're bald, for example. As a substitute, a thin stripe of paper would also work.
    – Alex
    Nov 6, 2015 at 14:14
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    Or, tape it at both ends across the door, and let the hair break instead of fall.
    – Carcigenicate
    Nov 6, 2015 at 19:38
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    Paper and tape are good means to make sure it stays in place but these also bear a risk that an intruder sees them and will restore them later.
    – Takkat
    Nov 6, 2015 at 21:08
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    If you have tape to make it stay in place, why not just use a thin piece of scotch tape? People are unlikely to notice it if you place it on the top near the door hinges.
    – corsiKa
    Nov 6, 2015 at 22:24
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    The trick is to place it in a specific way that will be broken by opening, and remember how it was placed. But you also have to take into account what will happen when you open the door yourself...
    – Dronz
    Nov 8, 2015 at 0:50
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Install door sensors. These work similar to the ones on a garage door, but you can buy them for interior man-doors. Some of them are very inconspicuous.

Wire it up to a Raspberry Pi or another device that can record when the door was opened and save it to a log. There may even be ready-made products that do all of this, but my inner geek would want to wire it up and program it myself.

When you return, simply check the log and make sure there are no unexplained log entries (i.e. nothing between when you left and when you returned).

If you cannot trust your roommates, a better option might be to move.

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    I bought this book a while ago and one of the projects is wiring up a simple home intrusion system. It is nothing professional-grade (don't cancel your ADT) but could be useful for something simple like this.
    – user2021
    Nov 6, 2015 at 15:25
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    Server running? What about breaks in power? `better option might be to move' - He mentions leave... which sounds Military... and probably doesn't have choice of room OR roommates. Also, I've lived with people I trust who have friends/family I don't trust...
    – WernerCD
    Nov 6, 2015 at 19:14
  • @WernerCD one can run embedded systems on batteries that last quite a long time. The option about moving is an aside, I do not know the asker's situation but is worth mentioning. I too was military, have lived with untrustworthy family, etc and I know how that goes.
    – user2021
    Nov 7, 2015 at 3:41
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Stick gum on several spots like light switch, door knob, drawer etc., whereever you think the intruder might touch.

Take a picture of the gum. When you return, if there is a finger print or if the gum is in different shape to that of your picture then someone has touched it. You know what that means, then, don't you?

With my approach, you not only can say if someone has broken in but you also MAY have their finger prints. If they manage to remove the gum to protect their identity, you at least would have managed to gross them out. haha

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As you're shutting the door, get it as closed as you can while still reaching your arm in, and set something on the floor (solo/dixie cup, nut, anything that will move easily when pushed by the door) right by the door.

When you come back and open the door, check to see if you're pushing it or if it's already been shoved from where you placed it.

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    I'd recommend against something like this since with a cup you could very easily reproduce this if you did break in. Unless you fill the cup with wine or flour or something which will cause permanent stain/messes :)
    – enderland
    Nov 6, 2015 at 15:45
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    @enderland but if there's lots of stuff piled on the floor behind the door (in its open position) how would the intruder know what to put back?
    – Chris H
    Nov 6, 2015 at 16:49
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    @enderland yeah! What Chris H said! Waddaya got to say to that!
    – Jimmy Hoffa
    Nov 6, 2015 at 16:50
  • @JimmyHoffa.... make sure to take a picture of it as you left it so you remember how it was? :)
    – enderland
    Nov 6, 2015 at 20:04
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    You can make it even better: Upon leaving throw a Handfull of random small things (like different colored wrapped candy) behind the door and take a picture with your phone. When you return, open the door a small bit and take a picture from the same position. If all candy is lying in the same spot nobody opened your door. If someone opens the door, he will move the candy by opening it. And even if he would be a spy and use a camera before opening the door, he will have a hard time rearranging the candy perfectly through an almost closed door after leaving.
    – Falco
    Nov 9, 2015 at 13:08
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As many people have seen Death note animation series so I am just taking about that scenario where he comes to know that someone has break his room and tried to come in. In chapter 16 of Death note manga there is situation when Ryuzaki comes to know that someone has tried to enter his room. But the fun part is how he did that .

It's great if you will read those images which I have attached to this answer it will help you for better understanding.

If you hate reading it try seeing this video:

Here is the link of death note series Episode 8 and if you will start seeing it from 10:00 then you will see the whole explanation about how he comes to know that someone has tried to come in his room.

Written explanation:

At first he sets one little paper trap on his door which he used as decoy.

The second trap was that the handle of his door. He purposely lowers the handle of the door by 5 millimeter while he goes out of his room but most of them don't notice this so if someone may have break his room then he will notice that the handle will go straight horizontally.

The third one is the pencil lead which he used to attach with metal hinges of the door.Whenever he used to close the door he put one piece of lead above the hinges and when someone tries to open the door the leads gets crushed but he usually pull the lead out before he open the door but then if someone will try to open it then it will break easily.

Advantage:

  1. 3 levels of security which anyone can apply.

  2. You don't need anything which is too costly because the things which were used in this scenario is really common.

  3. You can implement your own ideas on this whole plan by putting anything else.

  4. Never ending, you can use this again and again without any waste of money (except that pencil lead)

  5. Many people see that paper trap because its quite common now days and they get happy that they have identified that trap and then they try to put it back if they are trying to go into your room but most of them don't even think that there can be even second trap and they neglect that part which increase the chances that your plan will work and even if someone will come to know about the second trap which is quite rare or if by the way the second trap fail to work according to plan then you will have one last trap there which will not fail.

Disadvantage:

  1. The success rate for this scenario is quite high but still there are chances that if someone may know all these tricks.

Images taken from Death note manga series:

Chaper 16: Page 14

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Chaper 16: Page 15

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Chaper 16: Page 16

enter image description here

URL for these images are :

http://www.mangatown.com/manga/death_note/v02/c016/14.html http://www.mangatown.com/manga/death_note/v02/c016/15.html http://www.mangatown.com/manga/death_note/v02/c016/16.html

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It's a variation on the existing answers, but perhaps a worthwhile one.

Attach a short piece of string to the top of the door frame, towards the handle side (as opposed to the hinge side). It should be short enough that it's likely to go unnoticed, and it should ideally be the same color as the door.

When you close the door, place the string so it goes over the top of the door into your room.

If someone opens the door, the string will hang straight down, and when they close the door, it will be pushed to where it's visible from the outside. When you return, before opening the door, check if the string is sticking out. If it is, someone has been in the room.

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Spread flour (or talcum powder) across the floor - especially in front of the door (and under the windows if you are worried about that).

The state of the flour upon return will be:

  • Undisturbed - nothing happened.
  • Disturbed - if there are foot prints in it, well, someone stepped in it.
  • Cleaned - someone came in, disturbed the flour, and then noticed that it was there and cleaned it up.
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    I couldn't think of how to do this without disturbing the flour in front of the door on my way out, thus the dixie cup approach..
    – Jimmy Hoffa
    Nov 6, 2015 at 16:42
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    And how do you test if someone disturbed, cleaned and the reapplied? Can't be that hard to vacuum and sprinkle. ... Needs to be something added. Like a placed hair(s) or distinct clumps that are unique and can be compared with before/after pictures.
    – WernerCD
    Nov 6, 2015 at 18:53
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My first answer would have been to tamper-proof the door as well, but I like this one too:

Fill up a large bowl with an assortment of candy and leave it in your room. While the contents of the bowl look random and uncounted, you have, in fact, counted it's contents. To enhance the plot, share the candy with your roommates before you leave. Let them know that there is a huge bowl of candy in your possession.

And in the spirit of Jimmy Hoffa's answer, poison the candy.

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  • Then you come back to a room full of dead rats.
    – emory
    Nov 7, 2015 at 22:47
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    No need to poison the candy, just get a bag of sugar-free Haribo gummy bears.
    – user6687
    Nov 9, 2015 at 12:57
  • It has to be good candy, candy worth lying for.
    – Carl
    Nov 9, 2015 at 15:15
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From ubuntu forums

NE Key November 19th, 2008, 03:51 PM As you leave and close the door place a matchstick or similar inconspicuous object in between the outside of the door and the door frame on the hinge side. When the door is opened it will fall out. You will be able to check, before you open the door yourself, if the door has been opened while you were away.

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    Its also important to note 1) where it's located in the doorframe and 2) if it's the right toothpick. I could notice it fall and put it back up - but it's less likely to be in the same location. I could also replace it, so I've seen the hint to break the toothpick in half - so you can verify the half you keep with you matches the half that's in the door-frame (For the extra EXTRA cautious).
    – WernerCD
    Nov 6, 2015 at 21:00
  • Great points. Perhaps a crayon (something easily removable) that is similarly-colored with the paint/wood could be used to mark the location. Our hypothetical unwelcome guest would be fairly unlikely to be looking for that.
    – FuriousFolder
    Nov 6, 2015 at 21:05
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Besides obvious answers about Books, paper, hairs and stuff like using half of a toothpick...

Have you considered getting a simple alarm system instead of relying on cheap tricks?

I use SimpiSafe at home. Granted at $259 for The 7 piece start package, it's going to be more expensive than hair, a tooth pick or a bag of flour... At the end of it you'll have an actual alarm system:

  • No yearly agreement
  • $15-25 a month depending on options chosen
  • No reliance on local internet - alerts happen via cell phone network
    • Read: Works with loss of internet
  • Battery powered parts - including battery backup in the base unit.
    • Read: Works with loss of power
  • Alerts on any status change
    • Read: Someone trips the motion sensor? Turns off the alarm system? Opens a window?
  • Change different sensors to silently alert
  • Change different sensors to alert local authorities (or, in this case, most likely not)
  • Useful when you leave for work - not just when you are on vacation/leave
  • Applied via small screws/tape. no need for complicated installs.
  • Adjustable - get enough for one room... or expand to a household

If you don't like SimpliSafe, other options exist - all with their own pros and cons...

The main point to consider is a REAL alarm system - even if for a single room. Just because you trust your roommates, don't mean that you trust their friends/family. Wanting to protect your stuff while your gone is reasonable. Expand that to protecting yourself daily and find a reasonable option - not parlor tricks.

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Draw the blinds so the room is dark, and wire up some kind of energy usage meter to the lighting circuit.

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