Is this in a Corrado? Have you drained the water system recently?
You might have an air lock some where in the system if you have one it could be blowing the water out as the air expands.
I used to get them in my old triumph Spitfire (Hmmmm). Run it standing with the bonnet up until it is slightly warm (e.g 1 or 2 mins) and give it some welly on the throttle and see what happend to the reserve reservior.
On the spit it was easier - the cap was on the radiator - when I changed the radiator and refilled the system - gave it some welly - it took another pint of water to top it up !!!
If you have not drained the system recently it could be that you have a pipe which is only leaking once warmed up.
I once wondered why the fuel gauage on my Spit was going down so quick, stopped, and started it up again. On opening the bonnet saw that a fuel pipe had split and was spraying petrol towards the manifold - luckily i had an alloy heat shield on my K&Ns which had deflected it and there was enough length on the pipe to shorten it and reconnect it (Ah happy days...)
