Rocksteady’s director Sefton Hill has confirmed that there are still, 3-4 secret easter eggs left in the game which have not been discovered. He stated:
“Some of them we put in there that were really obscure were found within a week…but there are still 3 or 4 things that people haven’t seen – some of them very subtle things. There’s one that we had right in one of the first demos that we did that nobody has found – it’s in there if you look hard enough.”
Challenge accepted. I know more than a few people who will be looking over those demos again.