My car

There is a sensor that is sensing that there is not enough coolant in your engine.

Rather than ask us, after the engine has cooled, go and see how much radiator fluid there is in the overflow tank.

If the level is low, you have a leak. Look for a leak, especially around the water pump. If you don't find one, take it to a mechanic to have it checked.

Could be low on coolant. You will have to open the hood to check.

If I were you I would suspect the coolant is low or the sender has gone south. ;)

mike

As the others say.. the overflow bottle is either nearly empty or the sensor in it is bad.

I seem to be 'speshul'... every one of the 12 or so Fords I've had with the 'overflow' reservoir system has 'used' coolant.

About a pint or so every 10,000 miles

Assuming you have checked the obvious coolant level, I'd simply remove that bulb or the power to it. Many cars don't have that sensor, I've never needed it.