While in the course of the third chapter (A Burden of Blood) of Plague Tale: Requiem, you can look for hidden chests which fall into the category of collectibles. This page on the guide contains their list and locations where you can find them.
This chest is found early in the stage, after getting close to the city gates (screenshot above).
Drive away rats by igniting a brazier. Reach the stairs marked in the image above. There is a small side area with a box.
This one is found in the village while searching for the herbalist. Start near the carcass of a cow.
Look into the surrounding left-hand buildings- in one of them you will find the chest shown in the picture.
Secret Box #1
You can find the first secret chest, i.e. a closed workbench, in the ballista barn, which the heroes visit in the fort in the final part of the chapter.
Start by recovering Amicia's equipment, which is lying in the barn. Approach the closed workbench then. For this and all subsequent objects of this type, it is required to sacrifice 1 knife to open a workshop.