In the course of Chapter 4 (Protector's Duty) of Plague Tale: Requiem, there are hidden chests to find that fall into the category of collectibles. This page of our guide contains a list of them and shows their locations.
Secret Box #1
The secret is to be found in the initial part of the stage, shortly after the rat wave appears. Amicia needs to light the basket in the wheeled cart to start traveling with it around the neighborhood.
Push the wheeled cart to the left, that is, to the optional area. You need to reach the higher shelf marked in the picture and a ladder leads there. On top, you will find a closed workbench that requires the use of 1 knife.
Look for this one while getting through Tar Workshop. Get to its opposite end, but don't leave the building yet. Look for a ladder (screenshot above) and use it to reach the upper balconies which Lucas occupied earlier.
Go through the upper balconies (on the way you have to climb up the shelf next to the big barrel) to the place where the chest is.
Progress in the story until you reach a large campfire in the wetlands. There are numerous enemies here.
Start by exploring buildings on the left. One of them contains the find (screenshot above) but it is guarded by rats. To clear the area, e.g. mix Ignifer with tar to drive the rats away and be able to access the chest safely.