This Fiendish Puzzle Literally appears out of a thin air - Batman Arkham Asylum Riddle

ARKHAM MANSION Room: South Corridor Location: About halfway down the hall there is a vent near the ceiling on the left side that you can grapple up to and stand on top of. Put on Detective Mode and look at the floor, pointing west. The bottom part of the question mark is at your feet, and the other part is against the wall.