Final answer:
Jem's pants were neatly mended and folded over the fence, which frightened him as it suggested someone knew about his presence and silently returned the pants.
Step-by-step explanation:
The condition of Jem's pants when he found them was untangled and neatly folded over the fence, which was eerily different from the state in which he'd left them tangled and abandoned in his panic. This frightened him because it indicated that someone knew he had been there and likely witnessed the events of that night, and went out of their way to mend and fold his pants. This thoughtful act was incongruous with the danger he felt at the time, adding an unsettling layer to the incident.