“Google says it has fixed an issue that allowed old owners of Nest security cameras to continue to view a feed from the device, even after deregistering it from their account. The issue could have potentially allowed an old owner of one of the cameras to continue to look through it, even after selling it to someone else. The new owner would have had no indication that a stranger could be able to look inside their home. ”

Source : Used Nest cameras had bug that let previous owners peer into homes – The Verge