The above answer from wanderista is correct. In chapter six of Harper Lee's To Kill a Mockingbird, Jem and Dill plan to go out late at night and try to sneak a peak through the Radley's window. It is a rather bold move, but things do not quite work out as planned. Going into the Radleys' yard, the kids have to go under a barbed-wire fence; of course, they will have to do the same coming back out.
Unfortunately, though both of the boys are able to glance into a window as planned, Nathan Radley hears something (the kids) and comes outside with his shotgun. Naturally this scares the three kids and they race for the fence. They are frightened but they dare not stop. Scout tells it this way:
Jem held the bottom wire; 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.
By now a crowd has gathered because Nathan Radley did fire a warning shot, and Jem gets in a bit of trouble for showing up on the sidewalk without his pants. Soon Jem and Scout are in bed and Jem goes to retrieve his pants. This is a significant growing-up moment for Jem, as he decides he would rather face his fears than to face his father's disappointment with him.
Jem does speak about it that night, but in chapter seven he finally admits that he discovered something very odd when he got his pants. First of all, they were neatly folded over the fence, "like they were expectin' me." Someone knew the pants were there and knew Jem would be back for them. Even more significant, the torn overalls had been clumsily repaired,
"[n]ot 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.”
Jem shuddered. “Like somebody was readin‘ my mind… like somebody could tell what I was gonna do. Can’t anybody tell what I’m gonna do lest they know me, can they, Scout?”
Jem does not know exactly what happened or who repaired his pants, but he is beginning to realize that someone close to him, and therefore also to Scout, is looking after them.
It is clear that Nathan Radley did not sew Jem's overalls, so that only leaves one person who might sew as badly as he would--Boo Radley. If that is true, Jem has much to think about.
In Chapters 6 and 7 of Harper Lee's "To Kill a Mockingbird", Jem loses his pants after running away from the Radley home;
[Jem] "I’m goin‘ after ’em, [the pants]"
[Scout] "...You can’t. I won’t let you."
[Jem] "I’ve got to."
[Scout] "You do an‘ I’ll wake up Atticus."
[Jem] "You do and I’ll kill you."
However, very mysteriously, when Jem goes back to get his pants, he finds them neatly folded, as if somebody had touched them;
[Jem] "There’s something I didn’t tell you.'"
[Scout] "About what?”
[Jem] "About that night."
[Scout] "You’ve never told me anything about that night..."
[Jem] "...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—...When I went back, they were folded across the fence… like they were expectin’ me."
[Jem] "And something else— ...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—"