This may be wrong thinking all together but there are no leaks i would think that much missing would be noticed. No poor running no extra smoke... None in oil no bubbles in Over flow resovore? Have you tried to fill it up again and see if it stays full? What if while you were running it down the road lets say you lost some out of a hose that wasent sealed up 100% but now it is and or the leak does not happen at idle or till its ran or ran hard? Just a thought i had..