There certainly is a sensor on both front passenger AND driver's seat on both my current and previous Leons.
Neither the previous one (2014 SE Tech) nor the current one (2019 SE Dynamic) have any option code 9P* on the data label
As I replied to @tracktoy in my previous post, the data label does not include all the feature codes, just some selected ones. The full list of codes can be obtained from Erwin using car's VIN. I'm pretty sure that seat belt option code isn't included in the data label.
As for the sensors in front seats, it would be great if you could take a look under your front passenger seat first, then driver's seat. There should be a cable from sensor connected to the seat wiring loom, with a 2-pin blue connector.
Today I checked the passenger side of my car and saw the wiring loom under the seat and two cables connected to it, one from the seat base heater pad and the other from the backrest heater pad. There was no blue connector obviously. I'm going to check driver's side tomorrow.