This quest is not available in game.
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.