Hi Magnet,
Thank you for your full explanation of this issue as you see it. I have exactly the same problem on my A2 which I have lived with since I bought it new in 2004 (I know !). It;'s a 1-6FSi with around 70k on it now. I have NEVER found the source but fill it whenever the bonnet it off or when the warning appears.
My son now has the car for a while, and uses it more often than it used to be, but unfortunately is not as diligent as me and the water seems to be leaking far mnore often. I have replaced the overflow tank as there was evidence of weeping from the seam, but there are no other witness stains of anything leaking from any other covers or pipes. I have never found the plastic pipe behind the engine so I will take a look for that one as it is mentioned quite often in forums. I understand that as the engine coolant usually runs around 110°C so any water escaping will emerge as steam and there will never be a trail of red liquid to trace. This is also backed up by teh fact that the car never looses water overnight. Fill it in teh evening, and it will all be there the next day, but drive it once and half the water has gone from teh expansion tank!
