That is pretty common. A gas cap lock on any and all vehicles goes without saying. It both keeps gas from coming out, and foreign substances from getting in.
Of course, the thieves are punching tanks and slicing hoses going from the gas tank, but a gas cap lock at least makes them work for the fuel they are stealing.
Another suggestion I have: If you have a pickup truck,
buy a
tailgate lock.
The Master lock is the beefiest, followed by the Bully (although the Bully one is very difficult to install), followed by the McGard (which is very easy to get installed, but it is the weakest.) Even a hose clamp with a spot weld on the screw is a deterrent.
I am surprised at how often I find my tailgate lid down, but thankfully still there thanks to the lock on the hinge. I don't lock the handle because it is stupidly easy for someone to break that off and move the locking rod.