I worked in an office where food was being stolen every day. One guy who worked an early shift put his sandwich in at 6 AM and it was gone by 10 AM. No matter how many threats and warnings came from management (people would label their food saying they had "licked" it, someone put up a sign saying that one item in the fridge was poisoned, steal at your own risk), it continued.
My mother-in-law told me a funny story about the one and only office she worked in when she was very young. The boss felt entitled to take whatever he wanted from the fridge, including the secretaries' lunches. They finally got so fed up (no pun intended), that the gals baked some brownies with Ex-Lax in them.
You can guess the rest.