When you start the map and go through the first door, you can see a huge hangar outside, west from you. Going there, you will find a lump with 2 switches, the northern one opens temporarily a small southern door at the previous narrow hallway. Go through it, climb up the lift, open the door and get an unreachable shotgun atop some piled up boxes. Stepping onto the lift is what actually triggers the secret.
In the square hallways at the east end of the map, right past the gray UAC "desk", there is a green switch which lowers two walls and reveals two shotgunners. West of this switch is a small panel with black pipes, open this panel for a secret room containing a chainsaw.
When you reach the "fake" exit switch (the northern one), climb up to the tallest step and run through the west window, you'll fall in an outside area (it has red arrows on the ground) with a soulsphere (you must only reach it to lower the pillar it sits upon and a ledge that allows you back to the square hallways).