Before you take to locating the secret passage, you could complete the exploration of Xiao-Xiao's office. In the locked chest, you can find a Coin Purse - 35G and a Hand Mirror - 28G, and apart from that, you will find a Perfume Bottle - 7G. Finally, turn your attention to the safe shown in the above screenshot. Use the wirecutter to disarm the trap on the safe. When it comes to the correct combination, you can guess it, basing on the documents that you found earlier. The combination is 573. In the safe, there are a Necklace - 50G and a Pocket Watch - 20G.
Now, focus on examining the painting in one of the office's corners and, in the way that you have already learned, find and press the two buttons. Performing this action will result in unlocking the secret [The House of Blossoms - Secret Passage].