The sink periscope is placed inside a sink inside Barracks 2. It is made from the the sink's two faucet handles. The periscope, after the faucet handles are put together, would then exits out of a water-filled barrel that is placed next to Barracks 2, like a periscope that rises from inside the conning tower of a submarine. It is used by Hogan and his men for when they need to take a quick look around the camp to see what is going on when they are either confined to barracks or inside the compound at night.
There is another periscope inside the tunnels that is used to see what is going on just above the tunnels.