Although Mrs. Winterbottom seems just as upset about the lunatic as Phoebe is, the fact that she keeps this a secret from Mr. Winterbottom could suggest that she knows more about the lunatic than Phoebe does and is upset for a different reason. Her secrecy implies that she’s reluctant to be open with Mr. Winterbottom, perhaps because of his tendency to dismiss anything abnormal (like the note that Sal and Phoebe find) as irrelevant to their traditional, respectable family. The note contains
Walk Two Moons’s titular saying: it essentially advises the reader not to judge a person unless they fully understand that person’s experiences. This, of course, ties into the way Sal and Phoebe have judged characters like Margaret and the lunatic thus far in the novel. In this way, the note’s mysterious appearance seems almost fated to happen, hinting that the lesson it contains will be an important one for the girls to learn.