From my perspective which is viewed from the crew's side, a number of options are explored if a traveller has checked in but not shown up at the gate. Immediate check is made of the PNR file to see if they were intransit from another flight. If the flight is on the ground then most airlines will wait an extra five minutes for the poor victims to make the dash (providing there is more than one person involved; more consideration is also given for the last scheduled flight of the day). If a precleared customs checkpoint is involved they will contact those officals to determine if there is a hold up. If no clear explanation can be found then the passenger is deleted from the computer system and their seat is made available for a standby. If checked luggage is involved then the flight could experience a delay while it is being pulled. Without checked luggage, the only concern is that the head count on baord matches the number of boarding coupons pulled at the gate. Domestically a variance is allowed (up to 3 for my airline) but on international and transborder flights the counts have to match exactly. With no checked bags a flight is not held to look for one lost soul. Airlines know what the bad habits are of the FF . Policy can certainly vary per airline but the general response should be similar in the industry. Commenting on flight credits though is out of my league.