Full loss of coolant from expansion tank (unclear what caused the loss) - believed to occur over a couple of days.

Car drives normally at all times.

Temperature gauge reads normal. After a short spell of driving with boost (high rev range) the temperature gauge drops quickly - below the scale. Coolant appears to be boiling in the expansion tank.

Have checked for air in system. Doesn't appear to be any air (have checked bleed point and run for a while with expansion tank cap removed). Thanks for advice