Simple solution.
I also have an air card, and have had it for about 5-6 years, I have never had that problem.
When you leave your truck, "disconnect" from the air card server, when you get back in, "reconnect".
It only takes a very few seconds to "reconnect", in which case you are not open to anyone trying to mess with your computer.
You have been fortunate that all they are doing is 'using' your computer, they could disable it or crash it.
"Disconnect" - it's easy.
