10. Thomas Elliot Working The Arkham Night Shift
Long before he was revealed as the Identity Thief in Arkham City, and even longer before he enacted his (admittedly anti-climactic) scheme in Arkham Knight, Dr. Thomas Elliot - better known as Hush, to the heroes and villains of Gotham - was busy working in Arkham Asylum.
This particular easter egg doubles as a riddle, which can be found as Batman rescues Dr. Chen in the Asylum's Medical Facility. Elliot's schedule has him working long into the early hours of the morning, although one can imagine that wasn't all that he was doing.
Elliot's Arkhamverse origin was fleshed out further in Arkham City and Knight, and he can even be seen in the church clutching a medical box - presumably containing the faces of his victims used to piece together his new face.