After you've snapped a shot of 23 of the 24 of the Chronicle of Arkham markers, you'll need to track down the final marker. The key to solving the mystery of Arkham is the Warden, whom you last saw in the Security Control Room. Head to this room in the Penitentiary and enter the small central office area. Look on the floor for a circle of scribbles and etchings that say "BATMAN." Enter Detective Mode and take an Environmental scan of this to complete the final Chronicle of Arkham Riddle.