Use the Small Brass Key to open Bookie Herod's Strongbox.
Although no mention of Private Thorsen is found in these records, hidden within the binding of one of Herod's ledger books is a small, brass key.
And after further study of his notes, a few references to a "secure strongbox" are contained among various papers. The box is somewhere within the Stockpile - the cavern complex at the back of Kurzen's Compound.
If this secure strongbox is found, and the small brass key can open it, then perhaps the mystery behind Thorsen's detection will be revealed.