I get how frustrating that must feel. Unfortunately, Nest doesn’t give us a way to track if a camera has been re-connected on another account for privacy reasons. What you can do is check your Google Home app under “Devices” — if it’s still listed but showing offline, that usually means it hasn’t been factory reset or added to someone else’s account yet. If it’s completely gone from your device list, then it’s either been reset and set up elsewhere or deleted.
If you still have the receipt or serial number, I’d suggest contacting Google Support directly — they can at least confirm the status from their side, even if they won’t tell you who it’s connected to. And for peace of mind, I’d recommend removing it from your account and changing your Home access settings so no one else has lingering permissions.
