Jem loses his pants running away from the Radley property after trying to spy on Boo Radley. Later that night, he returns to find them mended and folded at the spot where he lost them.
When Jem, Scout, and Dill decide they want to see Boo Radley, they sneak onto the Radley property. Nathan Radley shoots at them -- unaware of who they are -- and they flee under a fence to escape. Jem's pants are caught on the fence. Scout says:
Dill and I rolled through and were halfway to the shelter of the schoolyard’s solitary oak when we sensed that Jem was not with us. We ran back and found him struggling in the fence, kicking his pants off to get loose. He ran to the oak tree in his shorts.
When they escape, they come upon a group of people discussing what happened on the Radley property. Atticus, Jem's father, asks where his pants are. Dill says he won them in a bet. Jem chimes in and confirms the story.
Not wanting Atticus to know they lied, Jem sneaks out that night to get his pants back. Scout waits nervously, saying:
There he was, returning to me. His white shirt bobbed over the back fence and slowly grew larger. He came up the back steps, latched the door behind him, and sat on his cot. Wordlessly, he held up his pants.
The next day, Jem tells Scout that when he arrived, his pants were mended and folded -- like someone knew he was coming back for them.