Isn't this as simple as UPS being private property? Therefore they have the right to prohibit firearms on their property and therefore on their property it is not a "legally possessed firearm".
Under Oregon state law, at least, the answer is "no". They are entitled to put up a sign, and they are entitled to ask anyone to leave who brings a weapon onto their property. Refusal to leave constitutes trespassing. The simple act of bringing a firearm onto their property in one's car is
not illegal
if one has a carry permit.
It only becomes illegal once they become aware of it and ask the person to leave and they refuse to do so.
Also...under the terms of the Western Region supplement language regarding "suspensions and discharges", there is a specific list of cardinal infractions that allow for immediate termination without the need to go thru progressive discipline (warning letter, suspension etc.) These offenses include drunk/drugs on duty, proveable dishonesty, theft, unprovoked assault on an employee, malicious destrction of company property etc. They do
not include having an otherwise legal gun in ones car in the parking lot. So...
if the employer were to somehow become aware of an otherwise legal gun in an employes car in the parking lot and
if they were to somehow get legal permission to enter and search that car, the
worst consequence that the owner of that car would face would be a warning letter.