Dash Red Light !!

Help help help please

Driving home today and noticd the red light on the temp guage glowing. Had to take a second look as the sun was out and the light was not  too bright. The oil temp stayed as normal, car drove as usual (slight hesitation at low revs as usual), fan cutting in as normal, temp needle did not go past the norm and when I got home the water level was still between min and max - anyone any ideas as to why the light was on?

Dodgy sensor, incorrect coolant mixture.

Arnold