In the Warden's Office, head back to the room with the Warden's portrait on the far wall. Turn left as soon as you enter the doorway and approach the wooden wall. You can destroy this with Explosive Gel. It may take a bit of trial and error, but one panel can be blown away. Beyond this is an unmarked room with a blueprint on the wall. It shows plans for the relocation or expansion of Arkham to Gotham City. This is a nod to the setting of Batman: Arkham City. For video and images of this Easter Egg, check out IGN's article, Warden Sharp's Plans For Arkham City.