SQL Server db user mapped to windows login not in SQL Server logins
The only way a login has rights to a server is if those rights have been granted.  The question is likely:  How did these rights get granted?

If DOMAIN\USER_A is a member of some Windows group, then through the rights granted to the group the DOMAIN\USER_A login exists in the server.  This is true even if there is no individual entry in sys.database_principals or sys.server_principals for that login.

You can look for the permission paths used by a login as follows:

EXEC xp_logininfo 'DOMAIN\USER_A', 'all'


You might find several permission paths for a login.