In (I think, it has been a while since I read it) "Surely you're joking, Mr. Feynman ", a collection of anecdotes from his life, he states he - to prove how inadequate (not to say pointless) security measures at Los Alamos were, he made a habit of opening safes he shouldn't have access to only to leave cryptic notes or objects in it for the safe owner to find and puzzle over.