I have a 1999 S70 AWD with 55,000 miles. The check engine light has come off/on many, many times throughout the life of the car. I've spoken to Volvo dealerships and independents. 99% of the time the light is due to the gas cap, not just filing up with the engine running. You see, the gas caps/filler nozziles are all, basically, defective. They must be VERY carefully screwed in every time (Volvo has never issued a recall). The slightest mistake will cause the light to come on. Usually, it will go off after 200-300 miles. If I could, I'd disconnect the light, but Volvo refuses to do it. My best suggestion is go to Volvo ONLY if it keeps going off/on for a long period. Out of warranty, you will go broke paying Volvo to "check the codes" only to tell you it is the gas cap. The only reason this light comes on is for an emissions problem. You can safely drive the car with the light on. Hope this helps.
PS: A word to the wise - At full service gas stations, always screw the cap on yourself., if not, you'll be looking at a check engine light for another 300 miles. I love my Volvo, but welcome to the world of European cars. This kind of stuff "just happens" with these cars.