I don't have a definite answer for you. Maybe it was some important evidence or a valuable item that drove the story forward in a way we can only guess at.
Well, that's a bit hard to say. It could have been anything from money and jewels to something more unexpected and symbolic related to the plot. The ambiguity adds to the intrigue of the film.
I'm not really sure. It's one of those mysterious elements of the movie that leaves it up to your imagination.