Many years ago a guy I worked for told me about a former laboratory co-worker who was notorious for getting into the company refrigerator during the night shift and helping himself to other people's food. My boss, good chemist that he was, got some phenolphthalein (which used to be the active ingredient in laxatives before being phased out due to concerns about carcinogenicity), sprinkled it on a plate of peanut brittle, then left it in the fridge at the end of the day. Shortly after that, there were no more incidents of food being swiped.
I do not recommend anyone try that, for obvious reasons. I'm just tellin' the story