Sorry, need to play the devil's advocate here. At first, I was going to say "Take a souvenir; that little headlight wiper." and then I asked myself who would do that.
I can think of a few people. My aging father for example. Do you think that every member of your family is as anal as we are about not having a single scratch on our painted bumpers? No. That's what bumpers are for. Sorry, but he's "evading responsability" because his bumper touched yours in a parking lot? That's a bit much.

That being said. I also check the my car when I leave, and that would really piss me off. On the spot, I would have done worse.