Why does Jem say that Boo Radley must not be home? Harper Lee's "To Kill a Mockingbird"

In Chapter 28 of Harper Lee's "To Kill a Mockingbird," the children depart for the Halloween pageant where Scout is going to perform.  "Thus began our longest journey," Scout narrates.  As they walk along, the street light casts "sharp shadows" on the Radley house and Jem remarks, "Bet nobody bothers them tonight."  The siblings laugh at their former fear of "haints," but Scout tells Jem to "cut it out" as they near the Radley home.

As they stand in front of the Radley house, Jem comments that no one appears to be home.  A mockingbird "poured out his repetoire."  And, as the mockingbird is a symbol of both Tom Robinson and Boo Radley, the significance of the hovering "mocker" cannot be missed after one has finished the narrative.  For, at this point in the story, Lee hints (foreshadows) the hovering of Boo Radley, who comes to Scout's rescue when she is attacked by Bob Ewell. 

Also, before they arrive at the pageant, Cecil Jacobs jumps out before the children, frightening them, thus foreshadowing the attack of Ewell.  The suggestion of danger along with the theme of bravery are in this chapter.  The little mockingbird is not afraid to sing all his songs, be they "shrill," "irascible," or melancholy--all the feelings of the recluse, Boo Radley, who will later emerge brave and heroic.