dont know if you tracked down the water problem yet but I had the exact problem in a dodge Rader( mitisbushi once. wouldfill the resivor tank up take it for a drive then checke the water when I got home and voila it was 1/2 gone. never a puddle in the driveway or any wetness in the engine bay. It turned out that I had a small pinch in the head gasket and when the vehicle got up to pressue the hot water would shout out as steam and evaporate instantanously. took about 3 months to track that gremlin down. vehilce had only 18k miles on it and it was a factory defect when the head was bolted on and the gasket shifted. at low presser everything worked just fine. had to find a mechanic that could run 60 miles an hour beside the car to spot the problem... hah hah.
collardog