Could anyone tell me if there is something to the idea that when the radiator/fan light comes on it means that the fan switch needs replacing?

I was in the parking lot of the Yachats Oregon post office and had just raised my hood to add (yet) more water to my overflow when this guy appeared out of nowhere. He said he had worked for years at a strictly Volvo wrecking yard and told me that if my light is coming on frequently, it means the fan switch is not working.

My common sense approach has been that there is a water leak somewhere. But, having owned 2 Volvos is my life, I know that there is another sense that must be used at times.

Thanks for any help offered.
Paula