I've seen that and I've even been under the map myself by accident while exploring whats accessible.
Basically, if you climb up the letter on the back of the warehouse the side with all the wooden logs. Just strafe from the edge of the ladder to the roof. Once there you've got an amazing camping spot.
As for getting out of the map... from the roof just walk or jump off the side the the roof. You'll end up under the map!
Every time Freight now comes up I either vote for the other map or leave the lobby.