There are lots fo reasons for missing disk space; most are well known.
Here’s one that I’ve just found and have not seen elsewhere.
I have 10 volumes I keep mounted (boot drive; photo work; video work; programming; data and so on) and a couple of shelves of bare drives I use for backup of all that, popping them into an external hot-swap enclosure as needed.
If any of that looks like what you do, then (based on yesterday’s experience) here’s a quick and simple check you might want to make.
Using Pathfinder (or however you make invisible files visible in the Finder – or the terminal if you prefer) visit the “Volumes” directory at the root level of your hard drive. (No: “Go” won’t suffice.)
What you should see there is a symlink to your boot drive and N number of aliases to other volumes you may have mounted.
What you should NOT see there actual folders which bear the name of your backup drive(s). If you see that, then the backup went to your boot drive, and not to the external drive you expected to use as the destination.
Until I discovered this “little issue” I was wondering where a significant chunk (113 GB) of my boot-drive space went.
Just “sharing” like a good Californian. 🙂