Usually this is due to a crack in the radiator from an accident or hitting a large speed bump causing a crack in the coolant delivery. Also, the fluid reservoir could be damaged and cause a leak.
Well the most obvious thing that I can think of is that there is a leak in the system. For there to be none in it I would say quite a big leak. Otherwise you may have a cracked head or block that is letting coolant burn off in the combustion chamber.