In the storage room right outside the projector room with the hostages, go to the corner. There is a door and three cabinets. You should be standing on the opposite side of the door, looking at the three cabinets. Look at the floor. The three cabinets will cast a shadow (if you have shadows on). Place a grenade directly on this corner and back out of the room. If placed correctly, when you re-enter the room, the three cabinets will be stacked horizontally, forming a blocked doorway. The enemy can easily shoot the top cabinet off, so defense is still required.