If you have taken the Western staircase, than, along your path to the upper level, [The House of Blossoms - Upper Level] remember to collect the Brooch - 25G that you find along your path. Taking this staircase will also allow you to approach a guard here and collect both of his Coin Purses - 5+9G, but it is a good idea to use the Focus here, or do that in turns, because the opponent does not spend too much time leaning against the balustrade. Apart from that, you should also watch out the bird cage and not move too fast around the cage, which would blow Garrett's cover. If you have taken the Northern staircase, you will have to spend more time and on approaching the guard, e.g. by crossing the abovementioned guest rooms.
Another upside of the Western staircase is that, in the spot shown in the above screenshot, you will be able to use the claw and start climbing. This will take you to rooftops of the Western guest rooms. Garrett will find here a ring (Serendi Stone Circles - Emerald Circle) and will also be able to jump down the hole in the roof, into one of the nearby rooms.
I recommend that you explore the main corridors on the upper level (while keeping appropriate precautions, of course!), as well as all of the four guest rooms located in the Northern part of this level. This will gain you, among others, two notes (List of Constellations and Coyle's Combination), which will prove helpful, while determining the combination for Xiao-Xiao's safe. Apart from that, you can obtain three Perfume Bottles - 3x7G, an Ashtray - 3G, a Hand Mirror - 8G and a Hairbrush - 4G.
After you have explored the upper level sufficiently well, I recommend that you return to where the guard, mentioned at the beginning of the text, was. Wait for him to stop at the balustrade, or divert his attention, with a throwable. Quickly, open the Southern door and, after you walk into the new location, close it. In the office, you can find a Hairbrush - 4G, a Strongbox Registry, a Necklace - 15G and an Ashtray - 3G. you can now use the lockpicks and unlock access to the eastern door to Xiao-Xiao's office. At this point, start by locating Madam Xiao-Xiao's Diary, thanks to which your objectives will update.