A lot depends on the specific circumstances and state law. Here in Indiana, we are not legally required to keep or guns concealed (Indiana law makes no distiction between open or concealed carry, a License to Carry a Handgun is required for either) and we do not have a duty to inform at traffic stops. Indiana law also prohibits, with a very few exceptions, employers from banning employees keeping firearms in their locked vehicles.
My personal policy regarding traffic stops is to not inform unless the officer directly asks me or there is a good chance that the gun's presence will become known. I've only been directly asked if I have firearms/weapons in the vehicle during a traffic stop twice and indirectly asked once ("do you have anything I should know about?") so it's usually a non-issue for me. Likewise, I rarely have anyone in the vehicle with me who doesn't already know that I carry or would be alarmed/upset to learn so.
For the sake of argument, however, I would simply inform the officer if directly asked about weapons in the vehicle. My safety and honesty when interacting with an officer of the law is much more important to me than the personal feelings of a passenger in my vehicle. If someone is unwilling to accept the fact that I lawfully carry a concealed firearm, then I don't much care to have them riding with me anyway.
Smith, and Wesson, and Me. -H. Callahan
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All bad precedents begin as justifiable measures. -J. Caesar