It probably flashes when you accelerate. Over the course of about
two years both of our Volvos (95 850 / 98 S70) will loose enough
coolant to do this. I just top it off (doesn't take much).

I think the fluid is just moving around a bit when you accelerate
causing the light to be seen.

The only known issue is the gray caps on the reservoir. There is a newer
blue colored cap that is less likely to crack. I think the gray ones
are more prone to a small crack/leaks. A tech once advised (on this
board) to be careful to not over-tighten the reservoir caps. My 98
came with the gray so you probably have one, too.