Many people are familiar with the acronym AWOL. It is the shortened form of 'absent without official leave' or just simply 'absent without leave'. The acronym appears to have originated in the United States military, referring to soldiers who were absent from duty without having had proper authorization.
The acronym is however in common use, and can loosely be used to indicate the absence of an individual or other entity either deliberately or unexpectedly, or for a host of other excuses.