When Jem went to get his trousers, they were folded and sewn. He didn't find them the way he expected to. In fact, the sewing job was terrible like a little kid did it.
Obviously this made him wonder a couple of things. First, he might have wondered who did it. And second, he might have thought that this person had to have been watching to even know that the trousers were out there. It would have been pretty dark and someone had to pay clear attention to know that they were out there.