Update: Google told The Verge that it has now fixed the problem.
We were recently made aware of an issue affecting some Nest cameras connected to third-party partner services via Works with Nest. We’ve since rolled out a fix for this issue that will update automatically, so if you own a Nest camera, there’s no need to take any action.
Used Nest cams allow the original owner to spy on whoever buys them. The problem was discovered by a former owner of the popular Nest Cam Indoor.
Worryingly, the behavior survives the official reset procedure, and right now there appears to be no fix available for it …