A few ideas for you:

1) Explosives--booby-trap your drawer to explode if opened by anyone but you.

2) Poisonous snake in drawer--place a poisonous snake in the drawer, it will then bite whoever opens it.

3) Private security--Have you considered hiring a guard (Former SAS are the best) to protect the drawer while you're not there?

4) Take the drawer with you whenever you're not present--might be cumbersome, but will stop the theft.

5) Cut off hands of everybody in the office, one by one. When the theft stops, you've found your culprit.

6) Ask people in your office to clean your drawer while you're out--nobody will go near it.

7) Lock the drawer

I can come up with more creative ideas if needed, but these should keep your office mates busy for a few days....

(The preceeding was meant as humour, and I take no responsibility whatsoever if you should choose to implement any of the above suggestions, however I do retain movie and publishing rights in such an event)