Velvet Assassin

After you exit the inn, you will come to a courtyard with a single guard pacing back and forth. Kill him and proceed down the corridor to the mill and you will see a door on your right. Enter it and look through the keyhole to the door on the right. You will see a guard. As soon as he turns away, open the door and kill him. Now, take the stairs down. Turn on the flashlight and look to the left. You will find a crate that you need to move. Once you do, smash the wood barring the doorway and go into the room. The list is on the desk in front you, and if you have done every other secret objective you will also get the "Miss Know-it-all" achievement. Before you leave the room, there are also a few items you can collect.

You can also follow the video from 3:40 if you don't feel like reading my description.