There are layers of permissions and privileges in macOS.
Of these some are accessible by unlocking Lock icon; and
use of your admin password to gain further admittance.
If you are curious but do not immediately choose to do some
action you may regret, there are articles to see relevancy in.
So if there were a hidden or unused admin level or former user
who hadn't been deleted, you could look to see if such exists.
(A previous Admin user may have been able to create a root level
access to Standard user accounts. ~ Whether or not this had been
done, could be a matter of conjecture. ..Or one of further study, in
deep command layers; as Terminal, with appropriate commands.
This should not be entered into lightly; damages could result.)
• Set up users, guests, and groups on Mac - Apple Support
https://support.apple.com/en-us/guide/mac-help/mtusr001/mac
• Delete a user or group on Mac - Apple Support
https://support.apple.com/guide/mac-help/delete-a-user-or-group-mchlp1557/mac
Not suggesting answers to complex issues, or hypothetical ones,
could be found by random looking. You could reinstall an all new
macOS system; or portion, if determined an incursion happened.
To reinstall using macOS Recovery can be tedious; and may be unwarranted.
Good luck & happy trails!🌞☘