This is the Directed 508D Invisibeam field disturbance sensor.

Suggested for convertibles and soft top vehicles.

Shock and glass break sensors both assume there's something solid for a thief to break. On a convertible with the roof down, or any vehicle parked with the windows open, there isn't. Someone simply reaches in.

The 508D solves that by emitting a radio wave and watching for disturbances in it. Reach into the field and the sensor knows, with no glass or panel needing to be touched.

Dual sensitivity zones. The outer field gives a warn-away chirp when someone gets close; the inner field triggers the full alarm when they actually reach in. Both are adjustable, so you can tune it to the vehicle rather than accepting whatever the factory setting gives you.

Also useful on a hard top when you want to arm the vehicle with the windows down on a hot day.