How can i stop loosing coolant from my car. Megane 03, 1.6 16v petrol?

I have been loosing coolant from my car, of about a ltr a week, just on short journys. When i have done 200mls plus i have not lost any. The temperature reading has been fine at all times.
The car is running great but i am a bit confused about what is happening.
I have checked under the car at regular intervals at all times of the day but there is no sign of water loss.
What should i do, its doing my head in.
Please help