I used to work in a three story building that only had one refrigerator for all 100 employees. Most of us (in our unit anyway) wouldn't use the refrigerator b/c 1.) there wasn't much room & 2.) once you put something in there it would get shoved to the back and you would have to hunt for it. Well, one of my coworkers did use it & she used it daily. She would go out to lunch a lot and then bring back her leftovers and store them in the frig for the next days lunch. She would always write her name on the box. She started getting the feeling that someone was nibbling on her leftovers. They wouldn't eat the entire contents but she felt as though some of her food was missing. She started monitoring this and sure enough someone was eating some of her food and then trying to cover it up with the remaining food so she wouldn't notice. So one day she decides to be clever and she writes a note on her food container stating something along the lines of "I spit in this food". Well, the person wrote back "so did I". Of course, she threw her food away. She never did figure out who had been eating her food.
On a side note, I don't think that pinterest idea would work in my office b/c if we see mold the item goes in the trash can! I once had someone throw away my slice of pizza b/c she was cleaning out the frig and thought it was old. It wasn't! I was p.o.'d b/c I had made that pizza from scratch and I wasn't taking a lunch break that day b/c I had to leave early for a dr. appointment.
I had someone accidentially swipe my lean cuisine before too. There for a while we were all bringing them in. Now, if we do bring one in, we'll write our initials on there.
I also had someone take my yogurt before and I threw a fit (I was preggo and again had to leave early for a dr. appointment). The next day when I opened the frig there it was! Someone had overheard me and put it back. No one ever fessed up so I threw it away. I figured I didn't know how long it had sat around before that person could sneak it back into the frig.