It was quite possibly the happiest day of Bond's life. When I first saw it (and every time since), my mind just went straight back to the scene on the beach that took place 15 minutes prior. The shell could be her way of saying, 'I loved you, too.' Why else would she keep a memento of that day? I think it would be some pretty elementary-level storytelling to have had the shell be an explicit clue, rather than just a memento from the beach. He checked her cellphone, and sure enough there was the message for him about Mr.
The realization that Vesper's feelings for him were genuine prompted him to think that maybe she'd left something else for him, too. She wanted him to know that it really meant something to her, and the shell signified that. I think Vesper deliberately left the shell in her bag so that Bond would find it, and remember that it was at this point he told her that he loved her.
