Hi Rabc.
Very frustrating when warning lights come on for no apparent reason :dry:
So long as you are sure that the system is not overheating then it is just a minor annoyance. When the light is on I think I would just check under the bonnet to satisfy my self that it wasn't actually overheating to the point that the light illuminates - hissing, clicking, ticking noises from the engine etc. Also that there has been no coolant loss when checked cold, if it had overheated it would have lost some fluid.
If all is well then your problem sounds like it is something to do with the sender to that light or a wiring connection to it. If you can find the sender it might be worth cleaning and remaking any connections. I've found an aerosol of electrical switch cleaner fluid really useful for this sort of thing.
Only my 2p worth but hope that helps :thumbs:
If it ain't broke..... Don't fix it!