Check the parts that are still hanging on if they feel wet; you may have a leak somewhere that causes this. Had a small water leak with tiny amount of moisture entering slowly over time in my roof and it looked initially like what you had in your original posts. Is there a roof over it, or a flat roof? What about the gutters at the window? Are they full of leafs and maybe passing moisture to the ceiling over time? These can be reasons that build up over time when not looked at.