But I understand the paranoia that something like that induces. It's not as funny the second time, so maybe the next time they unhook the brake cable, something like that.

But I agree that if you straighten up and act like it didn't faze you, the lack of reaction will make it less fun for the perpetrator and it probably won't happen again.

And just on the technical side, have you checked to see that the mechanism in your valve stem is all tight inside there? I've had it come loose and the air escape while I was in at work. You just need to use a small screwdriver or needlenose pliers to tighten it back up if that is the problem.