Along with what has already been said somebody just ran the fan to the AC in the back of the fridge and never put an on/off switch in the circuit. Take the cover off the outside of the fridge and look for the AC plug and see if it is plugged in there. Try unplugging the fridge at this location with it plugged into AC and see if the fan is still on.
Also are you sure it is a fan in the fridge and not a fan in the converter. The converter has a fan and it only runs when it is plugged into AC and the converter gets warm.