A number of years ago I was a camp counsellor for a cabin of junior high boys, and one of them stole the batteries out of my flashlight. He replaced them with his own almost-dead batteries, but didn't even put them in the right way, so I knew right away something was wrong. Further, I had marked every one of my batteries with my name, so it was pretty easy to track down who did it. Kind of sad, really, because I would have happily given him spares from my bag if he'd asked nicely.
Then one time my family was vacationing in Florida and went to Disney World. We had a small blue soft-shell cooler with a couple of soft drinks in it. It was put down for just a few moments, and I have the camcorder footage of someone walking off with it. At the time I taped it I thought, "Hey, we have a bag just like that..." and it wasn't until I put the camera down that I realized my family was searching for the cooler. I tried to run after the guy, but in that crowd, he was long gone.