Take the Muddy Note to Lieutenant Doren at the Rebel Camp.
Among the valuables of the strongbox is a muddy, scribbled note. Although barely legible, it looks to be an itinerary for Private Thorsen. It details his patrol routes, who he speaks with and how he spends his time with the rebels. It's signed with only one letter: -M.
But whoever wrote this note must have stayed very close to Thorsen. Whoever wrote this note is the traitor within the rebel camp.