I've grey boxed the first level of the first major mission of Dishonoured, I did this in Unity using probuilder.
It is not a perfect 1:1 model, especially not for building height. However, it does provide a good way of getting an overview of the general structure of the level and gives clarity on how a player would be able to navigate it. I used that clarity to write an analysis of the level, which can be read here.
Process
I started by defining a scale for the project. I did this via the blink ability of the PC, which is a set horizontal teleport ability. I used this ability to measure roughly the lengths of streets and houses. I then started modelling the level from street to street, making sure to include all relevant buildings and gameplay necessary elements.
I also made sure to include some of the level clutter and kept a strict colour code for the level, this helped me keep things visually clear.
I made sure to keep things as organized as possible via parent objects, and made prefabs of some commonly used elements, such as fences.
In order to minimise the number of objects as much as possible, I also made sure to ensure that each building consisted of as few parts as possible in ProBuilder, trying to build as many elements as possible into the core model.
For example, this building is essentially a single cube that I manipulated to achieve the desired shape rather than using several cubes to construct it.
A significant challenge was the internal parts of the level, which consisted of three internal areas that I had to model. However, I also had to integrate them with the general building. To address this, I took the approach of treating them as separate units.
For example, this interior, which I built separately from the building before joining them, is not seamless; however, for the purpose of this grey box, it was good enough, as it conveyed the general structure.