When I was in college, a girl in my part time work would steal food out of the refrigerate.
We couldn't figure out who it was, so we devised a plan. The night before fall quarter was over, we made a chocolate Stir 'n Frost cake with a big box of chocolate ex-lax and left it in the refrigerator. Sure enough, it was gone the next morning.
We found out the next quarter who the girl was... she had to drive from Rome, Georgia to somewhere in New Jersey! Her father wasn't happy about her need for the frequent bathroom stops.
Needless to say no food was stolen out of the refrigerator for the rest of the year.