Spooky Hidden Rooms
After you have ride through the airfield, and you have got the ladder that hangs out of the chopper, look with attention to the paintings that hang at the wall. When you shoot the paintings and all the officedoors, you will arrive in a strange room.

Secret Rooms
Level 2: Shoot all the highest windows in the far wall in the hangar. When you are in the truck, shoot all yellow barrels, and you will discover another secret room.

Level 4: When you are driving on the STAAR jeep, shoot every yellow box.

Level 5: Shoot the sign which shows the words General Weatherby on the second door right away when this door opens.

Level 7: Shoot all yellow barrels in the first area where the four purple zombies are throwing barrels.

Infrared
If you start the game, only shoot the first three STAAR team members. If you do this right, you will have an infrared sight.