I've heard that in some places (such as small hotels, guest houses, or rental rooms) some people are installing spy cameras (e.g. in form of fake smoke alarms). This problem affect people who're travelling a lot to foreign places and are worried about their privacy.
Here is one reference (Holiday cottage voyeur is jailed):
A man has been jailed for more than two years for setting up secret cameras to spy on people renting his cottage.
There are many variety of these, not only fake smoke alarms.
Are there any methods of finding these?