Right at the start, there is some sort of block in front of you. Go to the righthand side. The yellow wall has faded to a dirty white in a spot: a secret door. Inside you find some goodies and a switch.
The switch opens ...

...A secret door inside the stairwell leading to the red key. Outside you will find goodies. And somebody objecting to you taking them.

Not a secret: Inside the red door, before you enter the maze, walk up the staircase turning right. There is a nieche in the wall, with a flashing light. If you squeeze in here, a lift will take you down to perform a nice mafia-style execution of a couple of baddies. Better face the right way when you do.

Across the nuclear-waste filled moat from #3 (Did they build this place to the same safety standard as Windscale?) there is a faded panel in the wall, right behind some... leftovers. Secret door, leading to some goodies and a nice sniping platform.

Go back, open and enter the dark maze. Going as far left as you can get, there is a couple of metal stripes on the wall. Between the stripes, a secret door. Watch your six when you approach it.

At the opposite end of the maze, there is a U-shaped corridor with a low ceiling, guarded by a couple of Imps. In the middle of the outside bottom of the 'U', there is a secret door. If you used a exploding barrel to get rid of the Imps, the door may already be open.

Inside #6, you will find a chainsaw in the middle of a pool of waste, and probably some victims of your Mafia-style contract handling. Exactly two flights of stairs down from this room, the end wall - just before you turn right - is a secret room.