Don't you just hate it when someone at work helps himself to your lunch? Here's the solution:
Anti-Theft Lunch Bags are regular sandwich bags that have green splotches printed on both sides. After your sandwich is placed inside, no one will want to touch it.
http://www.skforlee.com/independent_work/lunch_bag.html -via Lifehacker
If a protected sandwich gets stolen, you can leave an actual moldy sandwich in its place. The thief will think
"Heh, I'm not fooled by fake mold!" and chow down.
My Dad claimed he replaced the whiskey in his dorm room with kerosene and waited in the next room to hear the janitor sneak in for a wee dram. He never lost any again.
If your work requires you to pass random drug testing, you can grind up some legal poppy-seeds and mix with the sandwich. Apparently these give a false positive. For the ethically ambivalent, you could insert actual material (horse tranquilizers, BC leaf, etc).
But how do you tell if your sandwich really has been in the fridge too long?