Secret Places (8)
When you start the level, jump through the vent and go right. Follow the path until you find the security screen. To the left of the security screen is a switch. Hit it and then run over to the other end of the room into the door that just opened. In this room is a hidden door as shown in the image.
When you come across the first mirror in the level, look for a crack in one of the nearby walls. Blow it up and jump in.
Directly across from this water fountain is an unblocked wall. Walk through it to find a secret place.
There's a secret door beside the regular door which leads to the red room.
There are hidden doors on opposite sides of the acid pool (two secret places). First hit the switch to line up all of the columns in the pool. Now go to the middle of each side and press on the walls to open them. One contains atomic health, and the other contains RPG ammo.
Continue through the level from the acid pool area until you get to the elevator. After going up the elevator, watch the left wall for a crack. Blow it up to find a jetpack and a secret place.
Once all of the vents are accessible, you can walk through them until you find this particular opening. When you step onto the lit floor of this vent opening, a door sound plays and a tiny door opens on a nearby wall (look for the yellow close to the center of the image). You need the jetpack to get to that opening.