An attendee can be marked absent automatically if they don't check-in on certain days or manually via the attendance manual update page.
By default if an attendee is scheduled to be present on a certain day and there is no check in on that day they are considered absent. This is an unexcused absence that is automatically generated and viewed as such in the herenow and herenow kiosk.
Attendees can also be marked as absent via the attendance module. Just like a staff person can create a manual attendance noting the checkin or out at certain day/times, an excused or unexcused absence can be created via that same mechanism in the neighboring tab: