What does Jem believe happened to Boo Radley?
This discussion pops up in Chapter 5, when Scout has taken to spending more time with Miss Maudie. The whole neighborhood gossips about Arthur "Boo" Radley's activities and speculates on where he is and what he's doing now, so Scout, Dill, and Jem get their fanciful ideas regarding Boo from listening to the adults around them (with a couple of notable exceptions: Atticus and Miss Maudie).
Scout and Miss Maudie are sitting on porch watching the sun set, and Scout asks her if she thinks Boo is still alive. Miss Maudie--as testament to her even-mindedness and refusal to stoop to petty neighborhood ways--corrects her: "His name is Arthur and he's alive." Scout asks how she knows and she says it's for the simple reason that she hasn't seen him carried out yet. This is when Scout says, "Maybe he died and they stuffed him up the chimney," an idea she admits she got from Jem--a boy not yet aware that such a move would drive the rest of the Radleys from the house.