2004 Jaguar X-Type Major loss of coolant

Car loses coolant slowly during urban driving but loses over a litre on a motorway run of approx 80 miles. No visible signs of coolant leak and car has been checked over by charles hurst jaguar. They carried out pressure test and gas test. Both proved negative. No loss of pressure and no change in colour during gas test. Is this an internal combustion problem, cracked head gasket? Where is the coolant going if not on the ground. Heater matrix fine. No wet floor panels etc. Would you suggest using something called K-Seal to sort out the problem? Grateful for response