In prerelease 11, all undiscovered traps are showing up as a blank unlit square. Particularly noticeable when I opened a door into a trap room and saw only the walls appear (so they aren't being treated as darkness).

Also it gets weirder yet. I triggered a fireball trap, and all of the fireballed squares remained with the * of the explosion (until I opened my inventory and forced a screen redraw, then they went back to empty squares).