I have had the red light flashing on the temperature gauge a few of times now (On cold mornings)
I checked the coolant level the first couple of times it happened and it looked fine as it was midway between high and low, so I figured it must be a faulty sensor.
After reading posts on this I opened the cap on the header tank and scraped the 2 metal rods gently with a flathead screwdriver. Even though the coolant level is way above the low mark there is still the top centimetre of the two sensor rods that aren't covered by the coolant so I expect its a combination of deposits on the rods and that they aren't fully immersed in the coolant.
If scraping the rods doesn't fix it then I will add a little more coolant.