Can you do the steps I posted above & tell us what you see…
- Look in /Users is there a 'deleted users' folder or any other user folders?
- What users are listed in sys prefs?
Do you know any file or folder names to look for? Or partial names?
The find command will look at everything on disk via Terminal, but you will need to use sudo (full access to everything), so you need to take care that any special characters are handled correctly. Anything that is not a letter or a number you need to take care with (but there are more exceptions than that). Please ask before you start adding extra characters - sudo used badly can ruin an OS giving you another problem.
The basic command is …
sudo find -x -iname username
You will need to enter the admin password to run the above command with full system access (& the password will be hidden - so you need to type carefully).
That will look inside the boot disk a.k.a. '/' for files & folders with the full text 'username', -iname means case-insensitive so UPPER & lowercase items will be found.
I'd suggest you start looking for the 'common home folders'…
sudo find -x -iname Documents
sudo find -x -iname Movies
sudo find -x -iname Music
etc