I have never, ever seen anything like that on the inside of the fender liner, on any vehicle. I agree with ricklack; maybe it came from the front side of the fender liner? Like some kind of bumper reinforcement? I did a Google image search for the part, and didn't come up with any exact matches. But some results did suggest that similar looking parts were some kind of reinforcement. If it were me, I would probably try removing the remaining screw to pull back the fender liner a bit, to see if it looks like the part should be on the other side. Otherwise, like "bagbvjw" suggested, try taking it to your dealer's body shop and they can probably identify it.