It does happen so if you don;t see him within a week, you can assume he is no longer with us. If you don't see him in a couple of days, you can try to dig him up using a small stick to run through the sand. Keep this in mind, sometimes the fish get their internal clocks backwards so they are up at night and hidden during the day. If you have to dig him up, make sure you feed him when you see him so he has a reason to be up that time of day. That helps reset the clock.
Hope this helps