To be "frost free" they have a heater which - every now and again - melts the beads of ice to allow the melt water to run away - you'll often see a V-shaped channel at the back - and a drain hole through to the outside. The water lands on the compressor and evaporates.
Unless the drain hole gets blocked with food debris. In which case the water forms a puddle at the bottom and comes out when you open the door.
Peccavi, August 2013
