Windows 10 camera window opens without initiating
Solution 1:
If this keeps happening even after reset and with no malware on your device, it means internal Windows app is doing it.
While there's low risk of privacy breach (they're not likely sending anything to anyone), it's still something better avoided.
Someone in a Microsoft forum thread said that in their case it was WavesSysSvc process:
I found out the application using my camera was WavesSysSvc. For some reason it was using my camera at those moments, so I ended it and disable it's startup. So far so good.
Might be your case as well, worth checking.