If you head down the hill by the allies CTF base, there is an 'L' shaped building. Go to it.

Near the building, there is a motorbike. Take it and turn left.

Drive towards the side of the building with a window and drive your bike toward the closed door.

When you get out, you should go through the door and be able to shoot out the window at others passing by.

If you do not get in, you are either too close to the door or too far away.

They cannot see you, but they can shoot you if they figure out that you are there. The only way to get out is to die (get shot or respawn).