2006 Citroen C3 engine temperature warning light on

After about 20 minutes driving the engine temperature warning light comes on with a warning to stop the engine immediately. This has happened 3 times randomly in last month. Each time after going down long but not steep hill, with air temperature about 0 to 2 deg C. Upon stopping the engine the fan continues running for about 10-15 minutes. The light and warning continue to appear if engine is restarted up until the fan switches off, when the problem disappears. Engine is clearly not hot as I can put my hand on cylinder/cyl. head, and exhaust manifold. Top hose is also cool. I hesitate to think it is the thermostat because it ought to overheat more quickly. Could it be an electrical sensor fault or perhaps the ECU getting confused?