Actually, I suggest marking it as containing a strong laxative, and see if it gets taken.

If it does, continue doing so (leaving it and marking it as having a laxative) until it stops being taken. Then continue to leave your stuff with the marking, but without the laxative, parhaps only occasionally putting the laxative in (be sure you remember which days you do that).

Or is that to Machiavellian?