How and when are Tom and Becky discovered missing in The Adventures of Tom Sawyer?

Tom and Becky are not discovered missing until church the next morning.  Although they did not come home after the outing the day before, their respective guardians both assume the children had spent the night at Mrs. Harper's.  Noticing that Becky is not at church, Mrs. Thatcher asks Mrs. Harper, "'Is my Becky going to sleep all day?  I just expected she would be tired to death'".  Mrs. Harper responds with surprise, not having seen Becky, and then Aunt Polly approaches her with a similar question about Tom.  The Harpers, Mrs. Thatcher, and Aunt Polly are by this time understandably worried, and ask the children and teachers at the Sabbath-school if anyone knows where Tom and Becky might be.  No one had noticed if Tom and Becky were on the ferryboat on the homeward trip the night before, and one man suggests the frightening possibility that they might still be in the cave (Chapter XXX).