"When I went back. . . they were folded across the fence. . . like they were expectin' me. . . And something else—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. . . 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?" (58).
The above passage shows Jem telling Scout more than just the facts of finding his pants mended. Jem realizes that it was most likely Boo who had done him the favor of freeing and mending his pants. This is ironic because previous to this episode, the kids thought they were spying on Boo, or at least trying to do so, but in reality, he was watching them. By watching the kids from his house, probably every summer—year in and year out—Boo would certainly get to know the personalities of Jem, Scout, and Dill. Jem must feel frightened about this situation, or at least a little taken aback, because of the irony.
Another thing that Jem discovers is that Boo understands what's going on in the neighborhood, with the kids, and life in general. He is probably more shy than he is crazy. Because of all the rumors and superstitions thrown around town about Boo, it would seem as if he was someone who didn't know the difference between right and wrong, or cause and effect. But as we can see above, he is intelligent and understands enough even to help others.
When Jem, Scout, and Dill were fooling around by trespassing into the Radley property, they got a scare when there was a gun shot. As expected, the children fled in a hurray. However, Jem was stuck. His pants were caught in the fence. So, he had to run away without his pants. Here is what the text says:
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.
Jem was afraid that he was going to get in trouble from Atticus, but he managed to dodge him. However, what made him afraid and perplexed was when he found his pants mended. The natural question was who mended the pants. The logical answer was Boo. That Boo would do such a thing frightened the children, because the implication was that Boo knew what they were up to. Boo was watching them.
When Jem goes back to get his pants at the Radley house he finds them mended. He expected to find the pants where he left them but instead finds them folded neatly and hung over the fence.
The mending was done by an inexpert hand, indicating that a man may have fixed Jem's pants.
This state of affairs is alarming to Jem because it signals that fact that someone (probably Boo Radley) knew that Jem had come into the yard and knew that he would come back.
When the Nathan Radley's shot goes off and the neighborhood comes running, Radley tells people a story that is not true. Jem is content at first to believe that no one knew who was really in the Radley yard.
Finding the pants mended and waiting for him, Jem realizes that someone did know who had been there and, beyond that, knew Jem well enough to predict he would return for his pants.
His adventure was not as secret as he had hoped. He now worries that Boo Radley knows who was attempting to spy on him.