Go to the building on your map that’s marked with a “?” and break down the wall.
Once inside, turn around and hit a question mark above where you came in to enter the next room. Start by throwing a Batarang at the Question Mark that's across the room, then run to the last non-green tile. Then throw another Batarang at one Mark to your right, and continue running when more tiles turn off. Throw another Batarang at the first Mark you threw at, and go to the end. Finally, hit the Mark to your left while facing the door you came in. Ride the elevator to the top, then glide to the Hostage for the rescue.