Coolant

Right clearly you have a coolant leak some where on the cooling system. When you topped the expansion bottle did you happen to notice where the fluid was leaking from? If not dont worry. Ok Now again top up the expansion bottle with fluid (water) and observe for any leaks. You should be looking around the front where the Rad, hoses from the rad to the engine, from the back of the engine to the heater matrix. If the leak is not to be found then my advice would be to book the vehcile into your dealership for a coolant leak.