Velvet Assassin

The prison commander's room is the room past the catwalk over the kitchen. When you enter you will need to hide in the cabinet in the back right corner to avoid being seen by the guard who will walk in. Once he leaves, exit the cabinet and creep toward the painting to the left of the door and you will get a "use" prompt. This will reveal a lever to pull which will open a secret passageway behind the desk in the back of the room. So, go to the passageway, take the ladder down, and creep through the hall. The prison commander is at the end staring at a fireplace and is stationary so you can simply creed up to him for the silent kill. There is also a collectible and a morphine on the table, and when you turn to leave there is a gun cabinet that may be hard to see in the corner to your right.

If you are having trouble finding anything based on my descriptions, skip to 4:45 on the youtube video and follow it.