Why does Users & Groups System Preference not display a user? There's a user from whom I inherited my work Mac who is listed under the /Users directory, however, this user doesn't appear in the Users &amp; Groups system preference. I am an admin on the Mac so I don't think that it's a permissions issue. Does anybody know why this may be happening?

FYI: My ultimate goal is to delete this user to free up disk space.
Most likely, the user was deleted, but the user's home folder was retained (See the option Don't change the home folder below).

It can be deleted if you don't need it. An offline backup of the computer or /users folder can also free you to delete the files knowing you could go back if needed.