Hi Dee,
You will find that every now and then a guest/traveller will not arrive. Sometimes it's because their travel plans have changed OR sometimes they are confused in regards to the dates of the booking and may arrive at a different date (which has happenned to me before.) OR they may arrive late into their reservation.
If the booking has passed and they have not arrived it is up to your descrition if you wish to refund the guest.
If you wish to make sure you are financially secured for all cancellations or "no-shows" I suggest using a "Strict" cancellation policy.
Cheers,
Nick