Your fear of law abiding citizens carrying around firearms may be a symptom of a pathological paranoia. I suggest consulting your Yellow Pages for psychiatric help.
If the employer has a posted sign " all vehicles subject to search" you DO NOT have the right that your car not be searched, once on the property, as a matter of fact your vehicle may be searched without your permission! Just ask anyone who works at a defense contractor!, nuclear power plant, or major port!
But that strip of rubber that touches his property gives him the right to set the rules on access and use of his property.
And if he says that you can't drive in with a gun in your car, you can't.
By the wauy, if you park a trailer on someone else's property without his permission, wgat are you called?