How about tuna or chicken salad sandwich with TONS of salt, and sugar in it
....Even if your not a fan of it, seems like a good way to hide a nice surprise
I was bored so I looked on google and found this:
But now you need some results. If you are comfortable with a passive-aggressive approach, try laying a trap. To your next pain au salade de thon (otherwise known as a tuna sandwich), add a dash of cayenne and half a scotch bonnet pepper. Not only will you be able to detect the culprit by his or her telltale body sweats and watery eyes, but you will also enhance your culinary reputation, since the scotch bonnet pepper, the hottest pepper known to humankind, is a favorite among today's up-and-coming chefs.
Also what kinda place do you work at? Office? and how big?
Had a thought that consist's of blacklight ink(Coat the brownbag with it), and a black light.
But I still kinda like the tuna or chicken idea