In the book To Kill a Mockingbird, what does Jem finally tell Scout about his pants that were retrieved?

Jem tells Scout that when he went back to find his pants, someone had sewn them up and left them for him. 

The incident with the pants is another turning point for the Boo Radley situation.  At this time, the children are still sort of afraid of Boo Radley, their reclusive neighbor, but they also are very curious.  They try to sneak onto his porch and leave him notes so that he will come out.  Dill is convinced that he is just lonely and needs good company. 

The incident with the pants occurred when Jem accidentally got his pants caught trying to sneak out of the Boo Radley lot.  He was afraid that Nathan Radley was going to shoot him, so he just left the pants there.  The children told Atticus and Rachel that they had been playing strip poker with matches.  They were expressly forbidden from bothering Boo Radley. 

Jem feels that he must go back and get the pants, despite the risk, because he does not want Atticus to know what really happened. 

He blew out his breath patiently. “I—it’s like this, Scout,” he muttered. “Atticus ain’t ever whipped me since I can remember. I wanta keep it that way.”

This was a thought. It seemed that Atticus threatened us every other day. “You mean he’s never caught you at anything.”

“Maybe so, but—I just wanta keep it that way, Scout. We shouldn’a done that tonight, Scout.” (Ch. 6) 

Scout is too young to understand that what Jem is really worried about is losing Atticus’s trust.  Scout is afraid that he will be shot if he goes back to get the pants.  Jem is concerned about getting the pants before anyone realizes they are gone.  

It is not until later that Scout finds out what really happened.  Jem went back to get the pants, and found that someone had left them there for him.  Not only that, but the person had actually tried, inexpertly, to sew up the damage. 

“When I went back, they were folded across the fence… like they were expectin’ me.”

“Across—”

“And something else—” Jem’s voice was flat. “Show you when we get home.  They’d been sewed up. Not like a lady sewed ‘em, like somethin’ I’d try to do.  All crooked. It’s almost like—”

“—somebody knew you were comin‘ back for ’em.” (Ch. 8) 

Jem finds this disturbing.  Someone knew he would come back for the pants.  However, when he thinks about it more, he realizes that it could only have been Boo Radley.  When they find the blanket on Scout’s shoulders during the fire, he begs Atticus not to return it, even telling him about the pants.  Scout is stunned, but Jem is more concerned about making sure Boo does not get in trouble.