coolant system light comes on in saxo?

my 2002 citroen saxo 1.4i coolant light comes on periodically, there is coolant in system, temperature gauge nver gets over half way, but never really rises from cold either. No pressure in cooling system when lid is opened and coolant is not hot. When light comes on fan kicks in and stays on for approx 10mins, this has happened on 2 occassions.