If you have a library, and something happens to it (it’s not mounted on boot, or permissions change such that plex cannot see the entries), especially if a large number change at once, plex should either mark the movies as unavailable, or move them to the trash. Provided the trash hasn’t been emptied, if the entries become available again, then plex should not have to re-scan all the entries.
This may also be the case if a file metadata is updated (not sure).
It could be useful to store a hash of the file, then as long as the hash matches, it doesn’t matter where the file is on the disk once plex finds it?
This came up when permissions on a folder containing about 100 movies were changed to 640 from 755, removing the execute bit from the directory. Plex removed the movies, then when the permissions were changed, it had to go back through and re-scan all 100 movies.