How do I find files that are missing from visual studio projects?

Question

Due to issues with merging etc, we have lots of project files that don’t contain all source code files that are within their folders.

Before I write a little tool, that

Answer 1

I've found the "Show missing files for VS 2019" Visual Studio extension. It will show the list both missing and not referenced from project files in the "Error list" window after the build.

For the older Visual Studio use "Show missing files for VS 2013-2017" extension.