A quick car search is really just for bombs, though if they've been told to I'm sure they would react to a gun if they found it.
I mean ... if you allow someone in with a car, with all the nooks and crannys and things that could be easily disassembled and hold a small pistol, there's no way you're really screening for weapons. I mean, you've have to strip a car apart piece by piece and spend hours going over it to get even 50% comfortable there was no gun hidden within. And then you'd have to run the driver through a metal detector right there.
From my experience most security is always to catch either really stupid BG's or honest citizens who didn't understand/forgot about something. i.e. they always make a big deal out of searching my car before I go on a military base, but it's not like they pull off the door panels or lift the rug in the trunk or check the spare tire space etc. Getting a gun past them would be no problem.
Or I could just bust in the head of the guy doing the search with a tire iron (they are always private security these days) and take his gun. Though of course this would reveal my intentions to the MP's patrolling within.