Put your initial 6 points to Spot Hidden and Investigations. You can't 100% either of them, so the chances of a lock not opening or an object not being visible is always there. If you don't find something, reload checkpoint. For example, the crowbar in mansion wasn't there for me despite Spot Hidden being at 85% at the time, but it was there after I reloaded checkpoint once. On the other hand I didn't put anything on Spot Hidden during my first playthrough yet the note on the dock was there for the first try despite people online claiming you need at least 60% for it. Everything can appear even if you don't put point into these skills at all. The chances are better with if you do tho.
Important to note: when you enter an area, a room, an investigation scene, etc. a green questionmark will appear in the bottom left corner if said area has hidden items. When you pick up all hidden items, the same symbol appears only with a checkmark instead of a question mark. Pay attention to that to make sure you picked up everything in an area.
Here's the description of what you find in which area of which chapter. Chapter 1, 6, 9, 10, 11, and 14 have no hidden objects.
CHAPTER 2:
1. paper in the little house where the alcohol is on the dock (1/26)
2. handle for the trapdoor left of the bar in barn (2/26)
3. ratchet for the trapdoor left of the bar in barn (3/26)
4. cogwheel for the trapdoor left of the bar in barn (4/26)
5. newsclipping during reconstruct in warehouse (5/26)
CHAPTER 3
1. shoe under chair at fire reconstruct scene in mansion (6/26)
2. bottle under table at fire reconstruct scene in mansion (7/26)
3. crowbar by the main door in mansion (8/26)
4. key on the shelf by sarah's room in mansion (9/26)
5. drawing in sarah's room between the sofa and closet in mansion (10/26)
6. prescription in sarah's room behind the dress cower in mansion (11/26)
7. diary under the bed in simon's room (opposed to sarah's room) in mansion (12/26)
8. stack of papers on the shelf to the right of the table in charles' office (where you chase the thief) (13/26)
9. paper between the chair and the table right next to the door in charles's office (14/26)
CHAPTER 4
1. during the first reconstruction the neck of the bald man sitting to the right (15/26)
CHAPTER 5
1. valve in circular reception building in the hall in asylum (16/26)
2. valve in storage room next to pharmacy, to the right from the administration in asylum (17/26)
3. police badge under the chair in the electric device room in asylum (18/26)
CHAPTER 7
1. amulet in the room with the safe in the corner under the bookpile in bookstore (19/26)
CHAPTER 8
1. wrench in the storage room in the hospital (20/26)
2. finger in a jar on the shelf next to table in fuller's office during the reconstruction in hospital (21/26)
3. medicine on the lower shelf during the reconstruction in hospital (22/26)
(NOTE: I got every item I mentioned previously, and the achievement only popped at the medicine. So despite saying 20, it only popped when I had 22)
CHAPTER 12
1. paper on the table opposite of the stairs in captain's cabin (23/26)
CHAPTER 13
1. paper in right room on the floor in the area you start in (24/26)
2. paper in the left room on the lower part of the shelf in the area you start in (25/26)
3. paper in the room on the floor you get in with the key on the top floor in the area you start in (26/26)