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.