Why does Miss Siphia really want her "Testament" from the church?

After the service and once they have all returned home, Miss Sophia pulls Huck aside and urgently asks him to return to the church and fetch her Testament, which she accidentally left there. Huck does as he is asked and finds the book, but also sees a note that has been slipped into it which reads, "half past two." Huck returns the Testament to Sophia, and promises that he did not read the note.