Absent subscriber is the carrier reason for a message that could not be delivered because the phone was switched off or out of coverage at every retry; the number is valid and usually delivers later.
The network knows the number but cannot reach the device. The SMSC retries during the validity period; if the phone never comes back, the message expires. It is nobody's fault and the number should stay on the list, unless it is absent send after send, which suggests a SIM that is no longer in use.
A high absent share on a first campaign to a region with poor coverage is normal; the same share on a list of active customers is a sign the list is old.
At SMSMeteor: absent subscribers are reported with their own reason so they are not confused with unknown numbers, and a longer validity on non-urgent content lets more of them deliver.