I must be one of the first people to discover this, because I haven't really heard anyone else mention it. So far from what I know, only 2 have been discovered one saying (as mentioned abover) I will return, while the other says something like You will pay for what you did. Not sure what it's in reference to, but small things like this just are always fun to see.
Another thing that seems to be overlooked that no one else I know has done is with Catwoman, at a certain point turn right and you'll be greeted to a rather awesome cutscene. Kupomogli gave me the heads up via PSN literally right when I was on that very part and I immediately called my friends that had already beaten it and they said they completely missed it.