I was replacing the passenger side speaker (a process I hope to never have to do again) when one of the nuts fell down behind the carpet along the wall beneath the dash. I stuck my hand down behind the carpet and felt something strange. When I pulled it out, it was an extra interior trim panel. The piece that rests against the armrest and that typically gets knocked loose when you slide your butt into the car. I have no idea how this panel got there, but it's brand new. I'm thinking it's been there since the car was built.