Could I be loosing the coolant in any other way.
Easy answer - Yes! It must be going somewhere.
Firstly, until the leak is fixed, check the level more regularly and top up long before it disappears from the header tank. Also find the bleed screws and make sure it is bled. This will ensure that although the level is low, it will still do its job, otherwise more damage will occur.
Possibilities:
1. External leak. You've checked all these, so we'll assume you've not missed anything.
2. Water pump. These tend to leak only when running, and often not until the engine gets hot and the system pressurises. The hot coolant will evaporate from the pump leak, so this is difficult to spot. Remove the cambelt covers and (with the engine stopped) feel around the pump for any signs of dampness or crusty deposits indicative of a leak. You may wish to run the engine for a few minutes, then turn off and check again. Careful, it will be hot.
3. Heater matrix. Do you get a sickly coolant smell inside the car? Otherwise difficult to diagnose until it makes the carpet wet. Like the water pump, this will mostly leak when hot and evaporate. Run the heater on warm and to the screen and see if it puts deposits on the screen. Otherwise may need a system pressure test to find it here.
4. Head gasket. Usually this will pressurise the system, so bubbles will appear in the header tank while running. Run the engine from cold and observe the header tank, see if it bubbles or over pressurises. This will also show if the pressure cap is not holding the pressure. Sometimes the head gasket will just take coolant into the cylinders and not pressurise the system. Spark plugs may show signs of rust! Again a professional pressure test may be necessary.
Hopefully this will help and others may add to/amend this list.
