What causes a gas leak in a fridgefreezer?

AEG no frost fridge freezer 3 and a half years old.  free standing, plenty of ventilation on both sides, never been moved.
Technician says has had slow gas leak, can't be repaired, not worth re-gassing as it will still leak.
What can be the cause?  I reckon it was a factory fault but AEG totally refuse to accept it.