I noticed my camera usage also (under battery usage, Android system) when I was troubleshooting my battery usage, when I had not used the camera.
I found that if you have Smart Stay turned on (under display settings, to keep the display from timing out while you're looking at the screen), it uses the camera for that feature/function.
If you turn off Smart Stay, you should see the camera battery usage diminish.
Not sure if that's actually answering your question tho.
Choose Cruz!