I think I found what you're looking for. In the process of going through the battery and rear camera guides, I came across a couple of pictures of what I believe is the part that fell out, pointed to by the red arrow.
[image|3545654]
Here's another view with the rear camera removed.
[image|3545658]
My belief this is the right part is reinforced by the fact that I don't see it in your picture, so I'm pretty sure that solves the mystery. Now, exactly how it goes in is something you may have to figure out for yourself, I'm afraid.