I park my bike every workday in a relatively busy UK city street where bicycle theft is fairly common. I use a very sturdy lock and always chain it through both wheels and a bike rack. Even so, I lost a bike last year a few blocks away, and a colleague reported having lost four (even old ones) in short order. So when I found that someone had looped their bicycle chain through my frame and the bike rack yesterday evening I was more than a little worried that someone did it so they could take their sweet time destroying the lock overnight (They didn't).
Since carrying around power tools and the receipt for my bike at all times is impractical, what can I do in this kind of situation?