Why is apple watch sensor still on even when im not wearing it?

So, i've searched this question in many different languages, but i still cant get the answer.

Im aware that the sensor on the back serves its own purposes, e.g., green for heart rate.

Also, when the light is on, it means these sensors are working.

Daily, apple watches could detect if it was being worn or not. In my personal experience, the success rate is significantly high.

The real question is, in such a case, why would Apple Watch still trigger on the sensors even if it was aware that it wasn't being worn? I can always find the watch pops out green light when it's alone lying on my bed, sometimes even in the middle of a charge.

Think i figured out the reason while i was trying to propose the question.

Apple Watch will also try to detect your health during sleep (could be others, but sleep is the most likely scernario), in normal cases it's fine.

But in some cases, ppl would set the strap or watchband to loose, and that loose could trigger the unworn state. Therefore, it doesnt know whats really going on but trying to find you UNTIL a threshold, such as absolute still for a while or just a few times of failure.

Sensors can not tell whether it's skin or not underneath it. lol. I bet it can get the read of heart rate even if u put some fluid in a bottom attached to it.

The origin of my question is brought by the tiny battery, im not happy with the duration (turns to ultra, somehow better) and when I see the power is being wasted because of the algorithm, it keeps making me wonder why and angry.