Twice last week someone stole my sandwich from the work fridge. I swear if I ever catch the person. https://www.youtube.com/watch?v=0BLflAZo8EU
You need to make a sandwich with some dog food in it. Guaranteed your sandwich won't go missing again.
If you happen to have some UV dye (I usually do), just put some on the plastic baggie and the sandwich. After it disappears and everyone is on break, just casually walk around wityh a UV flashlight till you arrive at the offending cubicle. Leave a couple of drops of deer scent on their chair.
http://www.dailymail.co.uk/news/article-2732898/Dear-Sandwich-Thief-Mans-hilarious-response-passive-aggressive-sign-posted-offices-fridge-door-sparks-incredible-kitchen-note-war.html