I'd go shooting again if i were you and set a trail cam looking at the vehicles and try and catch them in the act.
I actually do that when I go hunting or to a remote boat launch location. I park where there is a woodline close and I keep a Moultrie M80 black flash camera in the truck and I hide it where it will get pics of anyone around the vehicle.
You would be surprised at how many people walk by and look in a vehicle when there isnt anyone around. I always leave nothing in view in mine worth stealing.