If the car is unused for a few days does the level continue to drop or does the system have to be running/pressurised? It looks like it could be the waterpump which is driven by the timing belt and is behind a cover.Is it a diesel?Get the pump checked quickly because if its bearing fails it will snap the belt with obvious results.Another place is the heater matrix but thats normally accompanied by damp footwells&misting windows and the smell of hot antifreeze which is putrid.Hope this gives you some pointers.