[MWMan]

I had a "mysterious" coolant loss some time ago. I would go for ages without losing any, then all of a sudden I would get the tank coolant low alarm. After refilling it wopuld go for ages without loss and then lose the tank again. After a lot of searching (including dye in the system), it turned out to be the thermostat housing gasket, which only leaked when the thermostat opened. I generally only do short journeys.