Just by the shape of it, I immediately thought it looks like it could be a trim piece for one of the seat rails. Found this thread about seat removal in a search and it looks like the same part:
As already mentioned, it’s the cover from the end of the seat rail (the part the seat slides back and forth on) 510881347A9B9 is the actual part number, it should just push back on, or may even be surplus to requirements and there might already be a replacement fitted!