My guess is you may have a water leak somewhere. It could be the smallest leak that over years is dripping so slowly. I know someone who had to rebuild an entire bathroom after finding they had a leak in the wall for years. After workers took out wall they could tell how long it had taken to show up and then insurance paid for repairs because it was water leak.
Oh, wow! Cleaning out one of my clothes closets this afternoon, I had a large cardboard moving box in there with old purses, etc. When I moved it out of the way there is black mold on the walls it was touching, perfectly alligned with the box shape. So bleach is the next step I guess? Do you think I should paint it after belaching to seal? And my clothes will probably reek like bleach if I put them back in (and there are a TON of them)…oh, man, I need a DRINK!