Why do you think Twain leaves Tom and Becky’s whereabouts a mystery until the widow reveals that Tom’s gone missing? How does he then explain what happened?

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 says she had not scene Becky at all.

Because Twain wants to create danger and suspense by Twain making us believe Tom and Becky are at the Widows house. Tom is very dramatic when he explained.