We have discoloration on the ceiling that is directly adjacent to outside attic vents. They are exactly the same width and the lenth that they exentd across the ceiling is until you are tired of looking at it and paint. In a room with a 12' ceiling height, the discoloration appears at the 8' mark. The areas are not soft, which would indicate mousture. The discoloration does not smudge or wipe away indicating that it is not a mold of some sort. It does not react to cleaner of any kind. We have ask a roofer and a general contractor and they both just scratch their heads and say "What the heck?"