How would you handle a "food thief situation" at work?

We had food thieves in my workplace. One was getting into an office at night with a key and would take a whole pie leftover after we had a lunch or get into the lead’s desk drawer and eat half her cookies. I suggested making some prune brownies but she made brownies & ice them with Ex-lax melted in the icing. Maybe she shouldn’t have done that but it was in her private desk and in the drawer. The next day the woman called in sick saying she had caught a stomach bug. But nothing ever come up missing out of that office again after that. The lady told me what she did after she was retired and really don’t blame her. If it had been me I might have mixed some cayenne pepper into the brownies so that she would have gotten a hot mouth.

I have never experienced this but have seen some solutions.... mainly, make your lunch with intense hot sauce for a week. they probably wont eat your lunch the next week. or cat food.

or, get a lunchbag that is thermo controlled and just leave your lunch in your lunchbag in your car

Have a substantial breakfast, then you will not need lunch. Or, lunch on things that don't need refrigerating so can be kept in your drawer. Or, move the food to another kitchenette in your building. Or buy your lunch.

Make the thief sick then you know who it is

I always brought a lunchbox to work. I never needed to use the "common refrigerator".

I worked in a lab and I heard a story from one of my coworkers that at his old hospital one grumpy lady got very sick and tired of people stealing food so she wrongly left cookies out and she worked in micro and she sprinked some bacteria on the cookies.
Several people became very ill. She was charged.

I would place a label on it saying that I spit inside the food.