Final answer:
Mathilde notices that the necklace is gone the next morning.
Step-by-step explanation:
Mathilde notices that the necklace is gone the next morning. In the provided passage, it states that after the social event, Bernice climbs the stairs to her room and undresses for the night. At that moment, Marjorie comes in and apologizes for forgetting about the Deyo dance. This indicates that Mathilde realizes her necklace is missing when she goes to undress and sees Marjorie.