While going through the final stage 16 (King Hugo) in the Plague Tale: Requiem you can find the final tool chest, which is one of the finds. From this page of the guide, you will learn where the secret container is hidden.
The last toolbox is to be found at the very beginning of the stage, i.e. right after you start exploring the destroyed city. When you reach the stairs in the picture, first examine the building to the right.
The box is inside the building, next to the workbench.