The files that Drupal does not track
Any long-lived Drupal site, whether it stores files in a local folder or a remote bucket like Amazon S3, holds hundreds of files, and there is no easy way to see which ones Drupal still tracks and which it has lost sight of. The files come from four places: live content, leftovers from nodes deleted years ago, migration and backup residue, and direct uploads, such as a PDF dropped onto the server because it was faster.
This is a common problem in the sector, and it does not depend on the technology in use. Every content system faces it sooner or later. The root cause is two stores that must agree: the bytes in storage versus the database that is supposed to keep track of them. The moment both stores exist, they drift. It happens in three ways: records get deleted but the files stay, files get uploaded but never registered, and jobs fail halfway.
These accumulated unreferenced files are untracked files, a kind of dark data: information an organisation keeps but no longer uses.