What macOS hides
Purgeable space, local snapshots and Full Disk Access
What macOS hides
Two things use space and never show up in a directory walk, no matter how careful the walk is.
The first is purgeable space. macOS reports one number for free space and a larger one for space available if something important needs it. The gap between them is held by caches and snapshots that the system will drop under pressure. No file accounts for it, so Reclaim asks the volume directly rather than trying to find it on disk.
The second is local Time Machine snapshots. These sit on your disk even when no backup drive has been plugged in for months, and nothing in the file system will show them to you. Reclaim lists them through tmutil.
There is also a permissions problem worth mentioning. iPhone backups, Mail and Messages are invisible without Full Disk Access, and the obvious way to detect that, counting how many directories failed to open, does not work: hundreds fail on a healthy Mac regardless. Instead Reclaim tries to read one file that only Full Disk Access can reach, and shows a banner when it can't.
