It does not make my property yours nor does it give you the right to know what it is. You are wanting to extend your rights to destroy mine. If I park my vehicle on your property, by what right do you have access to procedes of the sweat of my brow. If you do not like that and ask me to move my vehicle off your property I will move it, but you have no right to access the contents of it.
Do you for one minute believe that this employer searched anyone's vehicle without first making random searches a condition of employment, or at least notifying employees of the possibility of a search taking place?