To check for any N codes in the register please go to Reporting > Reports > Attendance (tab) > Gaps in Register by Date:
Please refer to:
Once in this report ensure that in the top right you have clicked to show
To find N codes over a specified date range, please go to: Reporting > Reports > Attendance (red tab) > Unauthorised Absences
Select the date range required and click to show only unknown absences and choose.
This will show the student names, Year, form and date of the absence.