I'm with you. I was kidding and haven't ever done it. I definitely have seen it (the scenario went, "nice, thats matts rig, bet he left the keys in the tank and we can get shuttle done!) and heard of it and certainly don't leave my keys in the gas tank because of it, among other reasons.
I would however rather learn that it was the case instead of it actually being stolen, sold, trashed out, etc.