Could be similar in principle to taxi companies and taxi stands, predetermined points where cabs/taxis just sit and wait for calls in certain geographical areas.
"Ok, who's at 4..." <unit calls into Dispatch> "348 at 4" "348 head to <insert general location here> and call me..."
Dispatch calls out to whoever is at taxi stand 4, unit 348 calls in to say he's there, Dispatch tells him to go to the general area mentioned and then 348 will call back into Dispatch to say he's in that area.
I learned a long time ago that taxi companies have incredibly cutthroat attitudes so obviously they listen to each other's wide open communications: when the Dispatcher calls out for a customer they don't give specific addresses or else the competition would send a unit to the same location to beat 'em out of the business. So they dispatch a unit to the general location so he's "in the area" and then Dispatch gives the specific address so that unit can be right on it quick. Of course, that wouldn't stop a truly desperate company from still tagging along, but it seems to take care of the problems that might happen.
I'd guess that "security check" thing could be the same in principle, who knows.