As the Sergeant of the Guard on duty at the gate, I would follow the orders of the Initial Entry Controller to the letter. And, as such, we busted a lot of Command's kids driving drunk, plus some VIP's. Our arrest rate alone tripled the previous year.
If the FPCON states search every car, so be it. At the Main Gate at Ft. Benning the morning ingress numbered in the thousands of vehicles. I suspect a lot of other busy posts are the same, so threatening that a specific car would be searched and a weapon found is a bit silly. Most of the staff on post is civilian, and local and state laws are more likely their concern.
The shooting in Ft. Hood points out the problem -even soldiers on post aren't under a blanket prohibition.
http://kitup.military.com/2014/04/marine-corps-issues-interim-rules.html#more-27926
Note the Marines are escalating their policy, but Commanders will be Commanders. They love to micromanage these days, and the current Administration never lets a crisis go to waste. But, in in the past, when Commanders have attempted to prohibit weapons in private quarters off post, it was brought to Representatives. That isn't a phone call Commanders like to get.
Please note in the link the specific comment:
Currently, “there is no overarching prescriptive instruction adequately governing the use and handling of privately owned firearms aboard installations at either the [Defense Department] or Marine Corps level,” officials said April 3,"
And that just covers soldiers living on post. It very much is a patchwork of different views because state laws do have an effect on the local policy. Civilians are something else.
Could someone have problems bring weapons on post? If they were already doing something else that caught the attention of authorities. Most of the searches we conducted were caused by the smell of alcohol or marijuana rolling out the window as an ID was proffered, usually after 8PM.
A retiree coming on post to shop at the PX? Not hardly. If the individual has been coming onto post over the last few years, he already knows how it works.
What I wonder is why now be concerned, when the circumstances were much more serious numerous times before. What changed?