Secure the vehicle better when not in use.At least until the suspect current possible threat has passed. At least you saw them looking at it.

You might also secure a plate over the area with security type fasteners they cannot easily defeat. Otherwise with a battery powered recipricating saw they can probably remove it in two minutes. If you are the only occupant of a parking place. A heavy enough object you drive over. To deny them access to that area of the vehicle might be considered.Or dig two shallow trenches to drive into. Denying them enough room to get under the vehicle.Hard to say what is practicle and workable in any given situation.