The back room at Sidewalk is, unfortunately, a target for thieves. There was a recent thread over at the Olive Juice Board in which several folks mentioned they'd had items stolen, ranging from a wallet to a camera, to a guitar. When you think about it, considering the room is pretty open to anyone, yet folks leave their stuff all around, it's not that surprising that thefts take place there. Unless you come across someone going through your own bag, you really have know way of knowing if someone is handling their own stuff or someone elses.
It sucks that we have to be on guard in a place where we should be able to feel at home. It's a real pain, but I have been carrying my bag around with me everywhere, even when I go out front to hang out. I wish there was something that Sidewalk could do to make the place a little more secure, but definitely we in the community should make sure to let other folks know to be careful.