The camera is on all the time
That's what I meant: I don't know (as in I have zero clue) the camera is designed to operate that way. Is a naive assumption on my side and I'd be glad to learn this is not the case.
PIR sensor
No, I didn't expect a sensor, that's what I tried to say: the hardware is not there, so (on my mind) a constant image analysis/monitoring would be necessary in order to perceive movement and start recording, as in writing video to storage.