Fenix 7x measures pulse even on towel

That is a normal behavior with almost all optical sensors. Nothing new. 

How is that?

www.howtogeek.com/.../

Does not really explain how, just that it happens.

Oh, right. Some explanation can be found for example here:

https://consumer.huawei.com/en/support/content/en-us15801833/

Ok so it's the flickering of the artificial light, you could have just said that with a few words instead of linking to a long article. But thanks.

So that towel should be dead if it's just sunlight or no artificial light source and just dark.

The towel was never alive.

Many years ago I could point a traffic radar at a pop machine and get a speed reading.  The fluorescent lights in the cooler flicker, and the radar picked up the flicker as an indication of speed.

It's really no different than static on a radio. 

This is why tech devices have RFI emission limits and standards for rejecting interference.

Hello, it is not about the artificial light from the light inside the change room if you mean that. I am not going to explain how it was reproduced as it can be seen in the video. It is about code in the watch OS which need much improvement. Why cheaper devices as Huawei are smarter then Garmin i.e. ?The watch from the orher brand knows when is on your wrist and only then swith on the sensors. You even don’t receive messages from the phone on your watch when you don’t wear it, but Fenix keeps you notified even when you don’t wear it. So, a lot of smart features are not working in smart way for such expensive device as Garmin watches. It is only 1 or 2 years, till the other mentioned company will override Garmin if they go in that direction.