Genkidama, I think its either a bug that it does not go away when Enigma's sidequest is 100% complete because the PC version is full of bugs. Maybe it could be a reminder where it is I don't know. Another really annoying thing is that in New Game+ if you have already finished Enigma's sidequest 100% and go to the marker and shut down the system right away and open the hidden room (no riddler trophy inside) when you come out the Enigma datahandlers are still on the map which is stupid imo.
Yes, I also have the same icon marked on my map showing where Enigma's hideout is saying the same thing. I have completed all data packs relay's and finished singleplayer mode 100%. I went into the secert room and got the secert item too. I just want it off my map cause its the only thing left lol. Are you playing on the PC aswell?
I am having the same problem and I am playing on PC.