Speak with Corporal Sethman.
The note you gave me is signed with an "-M"...I don't know what that could mean.
But...the note also says this person shared patrol routes with Thorsen last week. So whoever shared patrol with Thorsen last week must be the traitor!
Go to Corporal Sethman. He handles the patrol schedules and can tell you who was with Thorsen.
Oh, yes I can check that for you. Let's see...last week it was...
...why, it was Private Merle scheduled with Thorsen. They had the same patrol times all of last week.
Upon completion of this quest you will gain: