This is a pretty good idea. Chances are if it is someone who is annoyed with something your doing and doesn't have the guts to confront you with it, the guy will easily be scared off by a note.

A while back I suspected the woman that used to watch our house was snooping around an area where I have my gun safes. An area where she had no reason to be while feeding our cats and checking our mail. The next trip we took, I put a note on the front of the safe that said "If you can read this, I can see you". Well, when we got home she was so freaked out by it she had left a note saying she couldn't watch our house anymore.