I had a roommate in college who kept doing this to me. I knew who the thief was because she was the only other person with access to the fridge. After I told her she needed to stop eating my yogurt, she said "You can eat mine" but hers had bananas in it, which I'm allergic to. I went several days without breakfast because of her, so I started writing my name on the container because she claimed she didn't notice the difference in yogurt containers. She took mine again. I wrote my name on it, put it in a paper bag, stapled the paper bag multiple times and wrote my name all over it. And then she got mad that I did that! You could put your food in a paper bag, staple it, attach bells to it. Then, if others are at the office when someone opens your food, they'll all hear it and see. This only works if the person is stealing during business hours, though.