We've taken a look at a few ways to handle kitchen thieves, like using fake mouldy sandwich bags. However, if nothing is working it's time to move to a more extreme measure: including laxatives with your food.
It's Evil Week at Lifehacker, which means we're looking into less-than-seemly methods for getting shit done. We like to think we're shedding light on these tactics as a way to help you do the opposite, but if you are, in fact, evil, you might find this week unironically helpful. That's up to you.
While not the kindest of ways to go after a notorious food thief, this method is effective at both identifying the culprit and catching him/her. The idea is to purchase chocolate-flavored laxative and either include it as-is with your lunch (as though you brought a chocolate bar as a dessert) or bake it into a brownie to help disguise the flavor. When the food thief eats it, they'll spend enough time in the bathroom to consider his/her actions and hopefully stop stealing from co-workers like yourself. This might take a couple of tries, but can be very effective.
Alternatively, if you're willing to ruin your food in the process, you can start lacing your sandwiches with extra-spicy peppers or other unpleasant tastes. You might have to test out a few options to find something your thief hates, but if it stops the stealing it might just be worth a few ruined sandwiches.