Easter Egg: So many in the Rocksteady developed games, with some of them being extremely well hidden - the most notorious?
Arkham Asylum featured a secret room featuring plans for the Arkham City complex. No-one ever found this area, either - Rocksteady outright revealed it's existence in a podcast 6 months after Asylum's release.
Arkham City featured a similar room, which a Rocksteady anticipated remaining hidden for some time - it was found within the first week of the game's release. However, the studio was adamant that fans hadn't found all of the game's Easter Eggs, and maintained this stance until 2015, when it was discovered that talking to Calendar Man with the console set to the day Rocksteady was formed would prompt a Leaning on the Fourth Wall instance with Calendar Man talking about the studio whilst never actually specifically referring to them. For those counting, that wasn't discovered for over 3 years after City was released.