That roof is just made for identification isn't it.
I think the "back gate" (as you called it) is more often used to send out the trucks that stock the "safe houses) than as a means of entry or exit for semi-undercover office workers.
The folks with exceedingly deep cover never visit the area, or are flown in from Eisenhower field at Fort Belvoir (20 miles or so to the South).
Not really. Most of the regular employees use that gate. Visitors use the front gate along with employees.