For Unit testing purposes I mocked some cpp files in a separate directory but still using the origin header files.
In my special case the cpp files (origin and mocked) are holding the member documentation.
Sadly the cpp mocks aren't documented. I've set INCLUDE_PATH to the origin of the h-files (where are the origin cpp files are located as well).
In the mocked cpp files I used @ingroup/@addtogroup to proper arrange them inside the modules section.