In reply to dakidunn: Not had a problem there myself, but usually have someone stood at the bottom. It's so sh***y at the top we argue about who's going up to rig!
We have never had any bother from anyone passing, most folk just walk past.
Unfortunately I think that you have been unlucky BUT you are right, it is a very public spot and I wouldn't leave anything in obvious view unattended there. Even at "climbers only spots" I don't leave gear unattended if anyone else is around.