Reach Chapter 2 until you have to crawl through the vents in. Immediately after the men talk about getting rid of a body in the furnace, the Thief-Taker General will whistle Frog's theme from Chrono Trigger.
A total of 73 secret areas must be found to unlock the "Finders Keepers" achievement. A metallic sound in the background will indicate that you are in a secret area, and in some cases you must interact with specific collectibles, objects, or loot to trigger the sound. To hear the sound better, lower the "Voice Volume" and "SFX Volume" options. Since secret areas are not listed in the statistics, you may want to start a new game and keep a check list. Also, not that dying before saving the game means you must visit the secret area again. For reference, the following chapters have the corresponding number of secret areas:
Grab one of the bottles in front of the "Reynard's Coffeehouse" shop (open world) in Clock Tower Plaza and aim into the air right above you. After you toss the bottle, you have 1.5 seconds before it hits the ground. Shoot an arrow when the bottle is close to hitting the ground. The closer the bottle, the better your chance of hitting it. You can also strafe left or right to adjust your aim if standing in place doesn't work. Save the game before throwing the bottle so you can simply reload the game and try again if you fail.
While inside the Stonecutter’s Court, find the Jeweler's Shop to the left of the guards that are chatting under the light. Enter the shop and knock out the guard to the left of the entrance, then turn to the right. Find the three cabinets below the red drapes and use a lockpick on the center cabinet to get the first mask. Once done, turn around and sneak into the cellar. Avoid the person working at the desk while moving to the rear left corner of the workshop. Interact with the painting and use the secret switch to unlatch it to find a wall safe. Enter the combination "739" to open the safe and collect the other item.
During Chapter 3 you will go through a hidden passageway within the brothel. From the peepholes that give you the secret code to unlock the door, go to the far right to find a peephole that reveals two people having sex.
Chapter 1: Lockdown is the best chapter to attempt the task of not alerting anyone since it is very brief and has few enemies. Note that if the eye-symbol above an enemy's head becomes fully red, it means they have entered combat mode and you are then spotted. If this occurs, just reload the last saved game. Always stay in the dark, don't knock out any enemies, and don't shoot any arrows. Remain crouched at all times and use the Shadow Dash to get from one dark area to another when enemies are not facing your direction. Explore the area to find alternative routes where less guards are present.
Upgrading the Focus ability "Dexterity" to its maximum leve is required for this. Focus upgrades are purchased from the Beggar's Queen in the northern-most part of The City. Once fully upgraded, go to the glowing gate around the corner to start Chapter 2 (can be replayed at the same location if you have already completed it). At the start of Chapter 2 is a guard with two pockets and a locked door. Activate Focus vision, and pick the guard's pocket, as well as the lock of the closed door to unlock the achievement. You can then pause the game and choose "Return To The City" to return to the Beggar's Queen.
Chapter 3 is the best chapter to kill/knockout 10 people using the environment. Enter the brothel and go down the hallway on the right to find a large opium machine in the center of the room. This machine can knock out everyone in the entire brothel if you manipulate it with the Wirecutter. Pick the locked door inside that same room to reach a room that leads to the area behind the opium machine. Find the panel on the wall and open it, then cut the wires. Once done, return to the room with the machine, and pull the lever to knock everyone out. Note: You can purchase the Wirecutter from any merchant in The City. Merchants are marked with a white bow and arrow symbol on your world map.
Scoring 5 million points is easiest in "House Of Bossoms" in "Chain & Gain" mode since it is a small map with plenty of loot items right next to each other. Start by stealthily killing or knocking out every guard on the entire map, qhich will deduct some points at first but make the overall process much easier. The goal in this Challenge mode is to collect all the loot in very quick succession--the faster you collect everything, the higher your multiplier and the more points you earn. Once every guard has been defeated, focus on collecting the loot without any disturbance. The loot always spawns in the same place so remember the locations. Go in every room, use Focus vision for a short moment to highlight all objects, and collect every item. After that, go to the next area until you have a chain count of at least 150. This will guarantee more than 5 million points, as long as you pick everything up fast enough and maintain your multiplier. This entire process will take about 18-20 minutes.
After collecting every item, go to the green door between the first and second floor to complete the mission and get a massive point bonus for your high chain count. You'll have 2 million points before completing the map but will more than double it due to the bonus points. Note: Most guards have pockets so make sure to grab their loot after defeating them.
Chapter 1: Lockdown is the easiest place to steal all loot and collectibles in a chapter. This chapter only has 60 loot items and 4 collectibles, which are pretty near each other and so you can easily find them by activating Focus vision. Loot and collectibles will then be highlighted in blue. Make sure to search every locker, drawer, and safe. You will also have to take a few hidden alternative routes and climb up some walls. Certain collectibles are hidden behind doors, so every time you open a door be sure to close it behind you and check to see if there is any loot on the ground. If you miss an object, simply backtrack or pause the game and select "Load - Chapter Start". In your Journal under "Player Progression - Statistics" you can see how many objects you have collected. Note that the achievement unlocks at the end of the chapter, not upon finding the final item.
It's possible to pick a lock with masterful speed at any door in the game that has three pins. Save the game before interacting with the door, then try picking the lock and remember the correct combination. Before you unlock the third pin, reload your saved game and simply enter the same combination as before in under three seconds. Just keep reloading your saved game until you are successful. Note: If you have already opened a door you want to try this at, return to the Clock Tower (your hideout) to have it become locked again.
In Chapter 5: Forsaken there are five golden pocket portraits you must collect within the asylum "Moira". They are found at the start of the chapter, where you have to search for the key to the female ward:
1. Go to the men's ward and enter the first hall on the right. Climb the tall cabinet on the left to find the first secret stash in an open vent.
2. After acquiring the first secret stash, jump across to the narrow ledge, and go down the hall to find another open vent with the secret stash.
3. Drop down and go to the back two doors of the second hallway of cells in the men's ward. The final door will be locked, but there is a hole in the wall. Enter the room next to it and crawl through the hole. Climb up to the vent above the blocked door to find the secret stash.
4. Go to the elevator shaft in the women's ward and shoot a rope arrow into the anchor point. Climb down and jump through the gap into the room below, then climb the crates to the left to find the secret stash on a pipe.
5. Climb out of the lower floor through the stairs, then turn left to find a guard room with missing bars. Crawl through it, turn left, and look behind the book shelves to find a closed vent. Unscrew the cover and immediately turn left to find the final secret stash.