The ring he’d found during his adventure occupied his mind. He couldn’t comfortably leave it home, and he knew instinctively he should wear it rarely. So, how would he ever explain its presence in his pocket? Hobbiton knew that Bungo Baggins had worn no ring. Everyone in the Great Smial knew Gerontius hadn’t left such a thing to his grandson.
When twice he realized it had fallen out of his pocket and he found it, once on the garden path and once in the square in front of the Green Dragon, Bilbo knew he must somehow keep it from escaping.