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.