Madame Loisel likely lost the necklace while she was hurrying home after the ball. The narrative suggests that she was flustered and anxious about the loss of the necklace after realizing it was missing. Given that she was traveling through the streets of Paris, it is reasonable to conclude that she might have dropped it on the way or perhaps it slipped from her grasp as she navigated the bustling city.
Furthermore, the story emphasizes her feelings of panic and desperation, which may have contributed to her not paying attention to her surroundings. The necklace was expensive and beautiful, and knowing it was gone must have been heartbreaking for her. This loss set off a chain of events that transformed her life dramatically, highlighting the themes of pride and the consequences of vanity in the story.