My friend lived in at the nurses accomodation, she put some bacon under the grill in the communal kitchen, when there were no clean plates she went to her room for one of hers, when she got back the bacon had gone! Anyway, my suggestion for yourself is leave a definate and distinct pattern in the butter, that way the guilty party will know you're on to them, think twice, and also hopefully be amused if irritated rather than go on about how petty they might think you've been to colleagues. HELP ME or DIE might be good.