Update: Google has reached out to us, and provided us with a statement on this matter.
“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.”
Apparently there is a bug within the Nest Cam software that, when you "factory reset" a Nest Cam, it doesn't really factory reset.
This means that if you have factory reset a Nest Cam and then sold it to someone, you can still see that camera, even though you no longer own it and it's no longer in your home.
Now that, is a big freaking deal.
According to the Wirecutter, who first broke this story, it appears that this happens when the original owner connected the Nest Cam to a Wink smart-home hub. This means that it isn't really a Nest problem, but more of a Wink problem. However, Nest should still be able to remedy the issue by simply disconnecting its products from Wink's smart-home hubs. Which is going to be happening this summer anyways, when the Works With Nest program dies.
Basically what happens is the former owner is able to see images through his Wink account. There's really no "hacking" needed to do this either. Just open up the app and you can see that the Nest Cam is still connected to your account. And that means you'll still be able to see that Nest Cam, even though it is no longer in your home. However, it does appear that when you factory reset the camera, instead of it recording video, it just takes photos every couple of seconds.
Since buyers will never know if the previous owner had attached the Nest Cam to their Wink account, it's impossible to know whether your Nest Cam has this issue or not. Which is why it's a good idea to not buy any used Nest Cams, at least until this issue is fixed.
Nest will be shutting down its Works With Nest program later this Summer, and that should fix the problem. As Wink will lose access to the Nest Cams and other Nest products in the process. This is all part of Google's new vision for the smart home and bringing the Made by Google and Nest teams together under one roof. And this shows that it is a good idea to shut down that program, as it looks like its partners haven't been doing a good job at keeping their firmware and software up-to-date.
This is a good reason why we caution our readers about buying used security cameras for their home (and why we almost never post a deal on a used security camera, on the site). It's because, you never know if the previous user is still going to have access to that camera, and can see what you're doing in your own home. Now if you got it from a family member or a friend, that's not as bad. But some random guy from Amazon or eBay, that's a bigger deal. Talk about invading your privacy.
There's no fix for the issue as it stands right now, so the best thing to do is not to buy any used Nest Cams, and it's a good idea to stay away from any used security cameras in general.