Note: If you check the ???, it says "A piece of parchment has been nailed here." If you select "Yes." to the choice whether to read the parchment or not, it reads: Mrahcva dauqe htsi yekllec eht. Vada uqons naem dnuos on. Read backwards (and respaced,) it translates to: "The cell key is the quadav charm. No sound means no quadav."
Once you get the item, trade it to the jail door at (I-8) (You will need to drop to reach the door) and trigger a cutscene with the merchants Serafin and Evrard. They will give you the Trader's Sack (key item) to return to Thunder Hawk.