What do the folded and sewn-up trousers make Jem realize about Boo Radley?

When Jem found his pants, they were all folded up, and the tears had been crudely sewn up, as if someone knew that he would be coming back for them. He finds this highly eerie. This makes Jem realize that they aren't the only ones who are curious. Boo is curious about them too.