Security :: DBA_AUDIT_SESSION Failed Login For Username That Does Not Exist

When reviewing failed login attempts with this query:

SELECT username, os_username, userhost, terminal, to_char(timestamp,'dd-mon-rrrr hh:mi:ss')
FROM sys.dba_audit_session
WHERE returncode != 0
and trunc(timestamp) >= trunc(sysdate-1)
ORDER BY 5

I find some records for a username that does not exist. In any of my databases. I presume that if an attempt to a nonexistent user was made, it would be rejected before it gets to the db. But then again, a bad password would also be rejected.