Passat coolant light is on

My VW Passat coolant light (the one that looks like a thermometer with
a wave under it) came on during the cold snap the other day and came
on again today (it's very cold out again today).

1) On both occasions the radiator was full and the overflow level was
where it should be in that plastic thing.
2) The car did not overheat. As it heated up, the temp gauge went up
slowly as it should.
3) When I turned the car off and on again, that light went out.

My plea for answers:

1) What could cause this?
2) What should I do about it?
3) Is this causing damage to my car as long as the car does not
overheat (or, can I just forget about it and drive as usual)?



Thank you for any assistance,
Judy