During the course of Chapter 6 (Leaving All Behind) of Plague Tale: Requiem there are hidden chests to find that fall under the category of collectibles. On this page you can find their list and locations where you find them.
Continue with the main story until you reach the quarry. Enter the quarry and turn right - you'll reach a locked pathway, but you can command Lucas to squeeze through a hole and open the gate.
Follow the newly opened path to reach the secret.
This chest is found when leaving the first area of the quarry. You'll reach a partially open location (screenshot above) - from there, you need to travel to the middle hill (marked in the screenshot above).
To get there, make sure to follow the path laid out by fallen trees. Among other things, there is a box.
Secret Box #1
You must leave the area where you found the previous chest and reach a larger building - the entrance to it is shown in the picture.
Once inside crouch down and go through the small opening marked in the picture.
You will reach a small room with an enclosed workshop. Use 1 knife to open it.
Continue with the story until you reach Dye Works - the place in question is visited in the final part of the chapter. The collectible is hidden in one of the huts on the right. To access it, you need to use Hugo's power so that the rats are commanded to leave the hut.
Now for gaining entry to the hut. Head to its back and hit a hook with the slingshot (screenshot above). This will open the way inside.