Answer by Kris Rosvold:
Put a fistful of chocolate flavored Ex-Lax chews in a plastic baggie in your lunch bag.
The culprit will uncontrollably "reveal" themselves within about 30 min. after dining upon your lunch, by spending the rest of the next day and a half in the bathroom.
My culprit was the boss. I don't think he ever took a lunch that he didn't pack in himself after that. Nothing was ever said to me about it, but he treated me with a good bit more respect and caution after that.
Yes, when it comes to thieves, I have zero sympathy and and huge mean streak… there were six or eight doses in that bag of "candy".
I specialize in assisting "quick karma".
Note: "I, of course, was just keeping them there because I am subject to occasional random constipation."
Ipecac syrup works too (especially in beverages), but doesn't have quite such a handy excuse.