Employees not reporting on duty without proper information is a matter of great concern. Try to find out from sources regarding his/her whereabouts, like friends, family, etc. You can also call his/her cell to know the reason. Sometimes, there are situations where employees are not able to inform their reporting supervisor, but this is serious misconduct on their part.
Once he/she reports on duty, arrange a counseling session and try to find out the reason for this irresponsible behavior. Moreover, serve him/her with a cautionary note. I do not think a warning letter should be served if it is the first time.
Hope this will help you.
Thanks and Regards,
Syed Yasar
Human Resources