Final answer:
Madame Loisel likely lost the necklace at the ball, where she was preoccupied with the festivities, and it was only upon returning home that she realized the necklace was missing.
Step-by-step explanation:
Madame Loisel likely lost the necklace at the ball, which is the event around which the pivotal moment of the story "The Necklace" by Guy de Maupassant centers. After attending a glamorous affair with her husband, during which she is adorned with the borrowed necklace, she returns home to find the necklace is no longer around her neck.
The text does not specify the exact moment when the necklace was lost, but it can be inferred that it happened during the course of the evening, possibly when she was most distracted by the festivities or on their way home.