Actually it IS meant to be the batsignal, Even Rocksteady says so, here is the part of the interview they say it:
Batman: Arkham City E3 2011
"So there is this rumor mill going on and even when you have done something in the game you will hear talk about, and maybe an hour later it has been mutated into something else. That narrative component is coming at you even when you are not in a narrative cinematic.
But the main way we keep the player focused is the Bat Signal and so if you need to know where the next mission is, just look in the sky. If you spend 15 hours tackling the Riddler challenges or teaming up with Bane, or doing side missions, actively ignoring the core narrative path, all you have got to do is look for the signal and you will pick up the path from there."
Here is the link to full interview:
https://www.digitalspy.com/gaming/i...steady-on-fan-service-open-world-setting.html