coolant leak

Hello to every one.
Purchased mini cooper in June 13 lovely car.have had issues with coolant,Only this week i had radiator replaced and had mechanic look over coolant bottle everything was fine then today coolant bottle empty that's two days after replacement,car going in on Monday morning,Would like to know any other reasons why this could happen,Have heard it may be head gasket but not sure .
Also car is not over heating.Any ideas?
Many Thanks