Ah, good point by Achim, the mystery may have been solved. The back rest of the rear side facing seat is fixed by one of these thumb nuts, located in the soft top storage well. I imagine, if it was loose, when you pulled the soft top out of the well, the nut may have been caught up in the soft top and somehow dropped off into the footwell, so it may have happened when you raised the hood, rather than when you lowered it. If you check the location behind the rear seat back you should find the bolt where the nut fits. The seat back will be loose at the moment. I may be letting my imagination run away, but i can't see how else it would end up in the footwell?