Mystery solved. As I was expecting disappearance of coolant has something to to with hot water system.. Well it was slow leak forum old partially damaged Bose right at engine coolant supply to hot water tank system outlet. Damaged part was cut, refitted and secured. Result, pressure was normal, those dry. No loss of coolant after running engine for 30 min. That was easy and not expensive at all Thank God.! Thank you all for helping me solve this mystery.