I just thought of this while watching The Incident. I am sure that I'm not the first to think of this. There are probably a hundred others. Anyway, I think Charlie's guitar is in the guitar case. I think that Jacob brought the guitar from the island to Hurley to convince him to go back to the island because we never really see what changed Hurley's mind. I think that when he opened the case and saw that it was Charlie's guitar, he knew, for some reason, that he should go back.
I haven't compared what the cases look like, because I really don't care that much, but I thought that this was pretty plausible.