I believe that it was the gas cap. I filled up today and tightened the cap appropriately. Since I was on my mobile phone ( I know Volvo drivers are supposed to be safe.) I didn't notice if the light was on after the gas fill. My drive was only a mile or so. When leaving this evening I noticed that the light didn't appear at or after startup. I'm assuming that the gas cap was it until it reappears. It is strange that the gas cap was the issue because it occurred half way through a tank instead of after the last gas stop. Thanks to all for your help on this one.