If you are skilled enough to find all of The Riddler's secrets maps, or if you can actually find all the Chronicle of Arkham pieces hidden throughout the game without them, there will be one more blank chronicle remaining to decipher. To unlock it, go all the way back to where you found Clayface behind the wall in the penitentiary. Go up the center stairs where you left Warden Sharp a long time ago, and there will be some scribbling on the floor. Use your Environmental Analysis on it and the last chronicle will unlock.