Following,
To make it trustworthy and clear, I created a test project (N5.5) : 7 files, 6 of 7 deleted from project and pool, one single project in folder. So, still 7 files in Audio folder, only one in project. Cleanup list stays empty.
Closing N5.5, opening N4.3.0 then. Cleanup, the 6 missing files are in the list !!!