I had a problem with trash cans disappearing. I set a box in the bushes that contained an old siren, a battery, and a micro-switch with a pull pin. A piece of thread went from the pull pin to the trash can. Next time someone grabbed the can the siren went off. I found the can laying there with the siren running and never had another missing can.
I should have tried that one with the bears!!!!
That, and a camera to take a quick picture... if I'd really wanted lots of pictures of the south end of a northbound bear!