Add to all the above that since he did take the vehicle w/o permission, he very likely could have been doing something that doen't quite qualify as totally LEGAL! In which case had he been caught, your vehicle could have been confiscated and impounded for a bit, and then there would have been the negative press.
Persoanlly, I think it might be time for an equipment audit just to be sure he didn't take anything else w/o asking.