I can’t imagine an employer of any size, say larger than ten employees, that would not have a statement in the employee manual that prohibits the possession of firearms on company premises, This would include the entire campus, buildings, parking areas, lawns and other common areas.
So there is a good chance that your boss is probably in violation of company policy. Check your employee handbook. An anonymous note to HR would be the right thing to do. Having a loaded weapon laying in an open space is an invitation to a horrendous accident.
As to legality, there are states with relaxed laws about where and when a gun may be possessed. But to the best of my knowledge, no state law would prevent an employer from enforcing an on-premises restriction against firearms.
On the other hand the parking lot might be a wholly different situation: I should add that I have read a case a while back that questioned the termination of an employee for having firearms locked in a vehicle in a company parking lot. I don’t know the outcome of this, 2nd amendment rights are a really touchy subject in this country (USA).
I do know that at the hospital where I work possession of firearms in the facility itself is grounds for discipline which might include termination
SRM