Boo can clearly be seen as a neighborhood observer through the gifts that he leaves for the children in the knot in the tree. It is evident that, although no one sees Boo, Boo sees what is going on. He may be locked within his house (by choice, it appears, at the end of the novel instead of as an imprisoned monster), but it is clear that he watches what is going on. In particular, he watches what is happening to Jem and Scout. When Jem tears his pants and leaves them behind on the gate, they are repaired and carefully folded for him when he goes back looking for them. Here, it is obvious that Boo knew Jem was in the yard and that he also knew that Jem would be in trouble if he did not get his pants back. Boo cares about the children for he sees in them a kindred spirit - he is, because of his mental handicaps, a child trapped in an adult's body. He wants to protect the children. For this reason, he knows that Bob Ewell poses a threat to them and he follows Ewell through the woods and stops his assault on the children.

In terms of a passage that expresses Boo's observational role, I would look to the aftermath of the attack and to the comments of Sheriff Tate and Atticus.