I'm not sure how I feel about the security having full armor. Yes, there are times when the security is outgunned, but there are also a lot of times when you're the only terrorist actually attacking and it's already hard enough to fight multiple people with mostly better weapons. I feel like the latter outnumbers the former. If this is added, then there needs to be some check on the power of the security- maybe by making the M4 less powerful, or by decreasing the number of security. I also don't really see security leaving to get armor too often. If they do, it seems like it's usually because they're half-dead. A better way to resolve the problem would be for people to realize that armor isn't all that necessary much of the time.
I think I remember the Enforcer and the FBI Truck being armor spawns. It was pretty cool. I would prefer Quido's idea of slowly regenerating armor, though it might generate reports. I have to mention, though- if you're going to bring back the armor pickup in certain vehicles, why not also bring back the health pickup for the Ambulance?
It would be particularly cool to make the back of the Enforcer/Ambulance regenerate health or armor at a faster rate than the driver's seat and/or have an initial health/armor boost that the front doesn't- then you could have someone acting as a traveling medic (so you get in the back and get 20 HP, then an additional 5 HP every 10 seconds or something).
My only concerns are abuse if the system is too generous and false reports. I think it would be better if it were designed with emergency use in mind for the back seating areas- a boost to health/armor, then slow regeneration. Maybe the initial boost could be greater if the passenger has really low health/armor when they get in the back (but not so much greater than people intentionally damage themselves). Of course, there would need to be restraints on the initial boost so people don't get in and out over and over.