Meng Yu had actually been peeping at the residents through the vent system. He frequently watched Mei Bao and saw Da Sen with her. He also tells them that Mo Li signed the rent contract with Mei Bao, not Da Sen. Detective Zhong looks into Madam Ye, the housekeeper, and finds out her victim is the one who hired her a lawyer. Furthermore, she had been hired by Mei Bao for the New Year's party. During that same party, Mo Li and Mei Bao come face to face while wearing the exact same outfit. The power outage was also on that same night. Soon after that night, Meng Yu stopped peeping at Mei Bao. (Source: Autumn carrot at MyDramaList)