In Chapter 18, what does Bertie discover when he takes the pearl necklace from the safe to Dahlia?

In Chapter 18, Bertie looks through the safe and finds a pearl necklace. When he goes to tell Dahlia the good news, he finds her in good spirits because she managed to get her pearl necklace back herself. Bertie realizes that the necklace he has actually belongs to Mrs. Trotter and goes to put the necklace back in the safe.