Confusing in most cases but .....no it's still just a simple personal property rights issue. Our, yours or mine , RKBA's does not trump our property rights if our firearm is kept in our personal (private property) vehicle.
... Where do ya draw the line ? ..... I believe the confusing issue here is commercial property vs personal property.
If your commercial property is gated, to control entry and clearly posted at the entrance that the following is prohibited ie cats , ugly trucks and firearms then cats , ugly trucks and firearms are legally prohibited regardless of state personal property or firearms or cat and ugly truck rights laws.
If your commercial property is not posted and entry physically controlled with a guard force or electronic security gates where anyone traversing your parking lot may come and go then they are subject to state and federal laws only with regards to what is or isn't contraband, allowed or prohibited in every personal vehicle.
Chicken little aside......that's how it is as I have enforced, encounterd and entertained it on private, commercial and federal property present and past.
Guess you have'nt read all the posts, the point being where the employer has a posted sign, or in the company handbook, that "WEAPONS ARE NOT ALLOWED" AND " ALL VEHICLES ARE SUBJECT TO SEARCH" does the employee have the right to search your private vehicle.