I had this same intermittent problem on my 99 S70.
If you open the coolant resevior cap (make sure the car is cold) and look inside there is a float which is like a washer that travels up and down on a spindle. If the float sticks in the down position it gives a low level light on the dash. Even though my coolant looed clean I think I had some crud on the spindle so that the float would sometimes get stuck. SolutionL I used a clean screwdriver to gently push the flat down and let it float back up (due ot the pressence of coolant - then pushed it down again and reapeated this about two dozen times. I think this loosened the crud on the spindle and since then has been just fine. Might not work in your case but worth a try.