Someone didn't do any house cleaning while they were living there is my guess.

My guess is same as maryland irisman, don't dust for a yr & really builds up!! I went to clean up old man's 2 unused bedrooms as they were going to have a reunion & relatives were going to stay in them. He had closed them day his wife died & never had opened them again. Lots of dust 1 window was rather drafty, I think it had small break & piece had fallen out. Also his wife had been sick for yr or 2 before she died so may not have been cleaned for up to 4 yrs. Dust balls, spider webs, dirt on window sills. It took long time & elbow grease to get them clean. Lot of pics of the family on the wall in 1 room so took each down & cleaned them also.