Riding the elevator up, on the way to the gold key, you better shoot this monitor. Well, if you want the goodies inside.

To get the goodies you can see on the left, run like mad up the slope, and jump at the top. Head-butt the fluorescent light fixture. It flickers and has lost colour anyway. A secret door opens right next to you.

You will find a room overlooking the exit slipgate. A target is visible over the door. Shoot the target, a room opens outside, containing a goodie and a slipgate back to the gold door.

After opening the final floodgate, you enter this flooded room. The pipe right in front of you leads to the exit slipgate.

Check the top area on the left. You'll find a dry room with some goodies.