Found the attached under passenger seat. It’s made of rubber. Any ideas what it is, if it is a part of the car after all? Cheers!

 

Oh yes … just realised it’s the rubber ‘grommet’ which goes under the seat plastic chrome lever, where the seat belt loop is! Realised because I found the other side one on the front seat when removing that lever for fixing! Now to remember where I put the first one!!