We use Monitoring plugin to check who's online from http://local/monitoring?part=sessions. And we found that if somebody use "Remember me on this computer", the user column will be empty. When he logout and login, the column data is correct.
And in the detail data, these users don't have ACEGI_SECURITY_LAST_USERNAME data (I guess Monitor plugin get username from this field?) But I still can see the username in ACEGI_SECURITY_CONTEXT.