newbdriver

Found this plastic clip on the carpet of my rear seat, I don't know if this popped out from some where. Any idea where this might have come from.

Gerf

Those are used on the underside of your door sills, the upper end fits in a groove on the bottom of the sill and the lower part snaps into the body panel. it's used in a few other spots as well like interior B-pillar covers etc

ESh

All the plastics around are held by these clips.

Did you remove any of the plastics/covers?
They can fell from the C pillar covers easily.