At the beginning of chapter 28, Jem walks Scout over to the school for the Halloween activities while also carrying her ham costume. As they go around the Radleys' home, Jem brings up superstitious ghosts like Haints and Hot Steams. He's trying to tease and scare Scout a little bit since it is Halloween and the house appears dark, quiet, and creepy. When Jem says "Boo must not be home. Listen," he points out the sound of a mockingbird, which may symbolize protection and safety. Another reason why Jem might point out the mockingbird's singing is, as Miss Stephanie Crawford says, Boo goes out at night and looks in people's windows. Jem could be suggesting (and teasing) that Boo is out on patrol, so the mockingbird feels safe enough to sing around his spooky home. The irony about this is that Jem is teasing. The children have long since dropped the idea that Boo Radley eats cats and squirrels, that he's the boogieman, or that he's dangerous. Scout even says the following:
"Boo doesn't mean anybody any harm, but I'm right glad you're along" (254).
Thus, Jem uses verbal irony as a joke to tease and scare Scout on Halloween, not because he really thinks that Boo Radley is a threat to them. Again, as for Boo Radley not being at home, Jem saying that he's not is ironic because he's always at home or in the yard.
On the way to the Halloween pageant, Scout and Jem must walk by the Radley house. Even though they know by now that Boo is harmless, it’s still a moonless, scary Halloween night, and the Radley house is dark and spooky. Jem says Boo must not be home because he hears a mockingbird singing in the Radley yard, an omen that Scout and Jem are safe going by the Radley house; this implies that a mockingbird would not sing its beautiful song anywhere around Boo’s vicinity. It’s also ironic that Jem hears a mockingbird singing in the Radley yard because Boo is symbolic of a mockingbird who never harms anyone or anything.
It is also ironic because Boo is always at home. Although, Boo does seem to be in the right place at the right time in regards to the children. He is there to wrap a blanket around Scout’s shoulders at Miss Maudie’s fire, he is able to place items in the knothole of the tree, and later he is able to save the children when he kills Bob Ewell. Being there for the children shows that he does get out and about town sometimes; however, for the most part, Boo is still a recluse. Luckily, he seems to keep an eye out for Scout and Jem throughout the novel, and this protective, kind side of Boo shows that he is certainly one of the main symbols of a mockingbird in the story.
It's important to the kids to know Boo is gone, because they are walking alone on a moonless night, and they're slightly spooked, Scout remarking, "It (the Radley home) is a scary place though, ain't it? Boo doesn't mean anybody any harm but I'm right glad you're along." Jem then goes on to re-state that Boo must not be home because a mockingbird is singing in the tree in front of the Radley house, "in blissful unawareness of whose tree he sat in." The irony of this development in the plot is that Boo Radley has been nowhere but home for years, and that the mockingbird becomes a symbol for Boo Radley after he saves the children from Ewell's attack.
It's important to the kids to know Boo is gone, because they are walking alone on a moonless night, and they're slightly spooked.