Happened to me for the first time a few weeks ago in CDG on AF. I had an aisle seat in PV and when I boarded, someone was in my assigned seat but the window next to him was open. I actually prefer windows but there were none left when I booked. The FA glanced at my PB and motioned me to the empty seat. Since I wanted the window seat, I didn't say anything, I just took it and was pleased to get my preference. I assume the guy next to me had the window and decided he wanted the aisle, which was fine with me.