Jem tells Scout that he found his pants waiting for him on the fence when he went to get them "that night."
When Jem says “that night” he is talking about the night he went on Boo Radley’s porch to leave a note and lost his pants. Jem, Scout, and Dill were always trying to talk to Boo Radley or make him come out. Boo Radley is the neighborhood recluse. They were very curious about him. That night, Nathan Radley thought Jem was a prowler and ran him off, but Jem’s pants got caught and he had to leave them.
When Atticus saw him without his pants, the children made up a story about gambling with matches and playing strip poker to explain the absence of his pants. Scout tried to talk Jem out of going back to get his pants, but he insisted.
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." (Ch. 6)
She thought he just didn’t want to be punished, but for him it was more than that. He didn’t want Atticus to be disappointed in him. This was the difference between his level of maturity and hers. He returned with his pants, patched up, and they didn't discuss it for awhile.
Later on, he finally told her what happened. He was trying to explain to her that it was Boo Radley that was reaching out to them.
"When I went back for my breeches- they were all in a tangle when I was gettin' out of 'em, I couldn't get 'em loose. When I went back-" Jem took a deep breath. "When I went back, they were folded across the fence... like they were expectin' me." (Ch. 7)
Boo Radley didn’t just leave the pants. He patched them up. Jem said it was like Boo Radley was “readin' my mind...” and could tell what he would do. He knew that Jem would come back for the pants. Jem realized that that tree’s knot hole was a hiding place, and there they found a ball of twine. It was the second gift that Boo Radley left them.
Jem’s pants are a remarkable overture of friendship for a man who nobody seems to understand. For years, Boo Radley has been the neighborhood spook. Adults avoid him, and children fear him. When the children reach out to him, he sees this for what it is, and reaches out to them.