The great thing about the Arkham games are the Easter eggs, some of them can be found by searching the riddles of the Riddler (confusing, isn't it?) but other ones don't have any clue left behind, they are just waiting for some player to find it like the one in Wayne Tower of Ace or the one of John Jones, Rene Montoya and Bullock in the GCPD building.
Most of the Easter eggs have been found, but recently, I was playing and I fell off the rooftop—I know, what a player I am!—and I saw a closed department, but the thing is that the department belongs to a friendly character by the name of John Constantine. Yes, it's him! The paranormal detective has an office in Gotham but it's closed because the police said that "mysterious things" happened there.
You can find it on Founders' Island, but it isn't a riddle or a clue. It's just waiting there for someone to spot it.
After all, it makes us think, why is Constantine in Gotham? What's he up to? Will he appear in a future DC game?