Be very careful
on what you think your boss can and cannot do.
They can have a company policy against employees carrying firearms at work which has nothing to do with posting the premises. While they cannot prosecute you for trespass or the violation, they can terminate you "for cause" which will cost you unemployment benefits in most states. If it is in their policy manual, properly explained/enforced and not restricted by some local ordinance you won't win a contest.
Be careful on the search and seizure rules too. Being prosecuted in court isn't their goal. If my company prohibits possession in my vehicle, I would park off their premises in a public area and walk into the building/facility. Of course, you can take your chances on discovery as well.
Many people like to talk about what their company/boss can't get away with and are surprised when they don't have a job anymore. The fact is, unless you are covered as one of the protected groups (gender, race, age, etc.) you can lose your job and usually unemployment benefits for violating company policies, no matter how unfair you think they are. By accepting the job you accept the rules which are not illegal. Even a strong union has no effect if they have approved the work rule. Your only hope there might be if they have negotiated the search and seizure right by contract or been involved in approving it in the work rules, then they might help you if the process was not followed.