Alarms get tripped so often by people bumper car parking, or by vibrations from a bus going by that it's pretty much worthless around here.

The sensitivity of the alarm is adjustable. In your case, there is no reason to have a hair-trigger response. Set it so that a major impact is needed to trigger it, but also set it so any door, trunk, or hatch being opened will trigger it. They can't steal your stuff without opening a door.

My alarm is set up so that just moving a door handle sets it off, but I can shut my tailgate with impunity.

tanstaafl.