So an employee in this case is not the same as a customer correct? Could a customer be subjected to the same search?
Yes.
You may leave my property to avoid the search, and you can refuse to do business with me if you are in disagreement with my policies.
If I post signs on the entrances to my lot letting you know that guns are not allowed on my property, and that I retain the right to search vehicles parked, and you enter the lot, then by YOUR action, you accepted my terms.