Can you please i.d. this part?

[coastk9]

2019 RX 350 F Sport.  Found under driver's seat.   No seat hardware or missing plastic hold downs apparent.  Can you help find it's origin?

[Lexus Owners Club]

That looks like the cover over the end of the seat rail
If they are all present and in place then it may be one that was dropped during the production of the vehicle and it was too difficult or takes too much time to retrieve.