From the book:

No one intended to leave Martha alone that afternoon, but it happened that everyone was called away, for one reason or another. Mrs. McFarland was attending the weekly card party held by the Women's Anti-Gambling League. Sister Nell's young man had called quite unexpectedly to take her for a long drive. Papa was at the office, as usual. It was Mary Ann's day out. As for Emeline, she certainly should have stayed in the house and looked after the little girl; but Emeline had a restless nature. "Would you mind, miss, if I just crossed the alley to speak a word to Mrs. Carleton's girl?" she asked Martha. "'Course not," replied the child. "You'd better lock the back door, though, and take the key, for I shall be upstairs."