Hi, If your coolant is going down, it can be only 2 things an external leak or an internal leak. Check the inside of the oil filler cap and/or the dipstick for 'Mayonnaise' Brown/white sludge on either of these probably means there is a problem with coolant getting into the engine. If you have checked everywhere and cant see any obvious external leaks, check the Y shaped metal coolant pipe which is sandwiched between the exhaust down pipe and engine block. This particular pipe corrodes easily and its difficult to see due to its location. The pipe may be weeping or dribbling out and where it is situated between the hot engine and exhaust, the coolant is maybe evaporating before it shows an obvious leak on the ground. Or its getting splashed on the hot engine/exhaust when the car is moving, hence the difficulty of spotting a leak.
Remember that when the engine is running the coolant is under pressure, so it is easier to find a leak. When the engine is cold obvious leaks are difficult to spot. If this particular pipe is the cause , you can buy one online for about £30. Hope this helps.