Which distance sensor to lock screen if I leave my seat?
Which sensor could be used to detect that I left my seat?


There is no sensor in any Mac that can be used to detect if you left your seat.

Rhetorically speaking, how would this hypothetical sensor work?  How would it differentiate between not being in your seat because you left and not being in your seat because you have a standing desk?

The answer is it can’t.  

The closest you can get to automating the locking/unlocking of your based on your physical position is through proximity.  There’s an app called Near Lock that purports to achieve this, however, it requires an iPhone; it’s implied that you use an Android as you mention a Huawai smartwatch in your linked question.  

Further, you’re looking to automate something that by design should be a user interactive process.  As someone responsible for security and IT governance in my organizations, I can clearly see why they have the policy they do.  They are trying to develop security minded habits.