The dark part of this wall is smooth - the rest has rivets. Shoot the smooth part. Secret inside.

In the room with the big acid pool, jump down next to the exit doors. If you fall in the acid, there is a lift underneath. Goodies inside.

Same acid lake, just the far wall. Jump in the acid here, swim into the tunnel. There is a secret room inside.

This may hurt.

These computers stand right next to the gold door. Get on top of the pile (always a problem with computers) and shoot the misaligned part of the wall. The exit from this secret room is right next to the gold key.

Right under the gold key is a secret - a Quad damage. Jump over to the ledge to get to it. The jump is not easy, but if you knock your head and fall a floor down, the plate marked on the floor is a lift. Ride it up again, and jump from it.

Use the Quad to eliminate all resistance on your way trough the gold door.