What is assinine is the employers assertion that an empoyees car ceases to be the employees just because of where it is parked.
Why does this have so much trouble getting through to you? It's your car, and you have no right whatsoever to put your car on his land. Oh, unless he says you can, of course. He isn't seizing your car; he's doing the equivalent of putting a guard and gate on the lot, and having the guard let some people in, and turn other people away.