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.