The check engine light can come on for a variety of reasons, one of which can be caused by not putting the gas cap on properly (Presurrized system requires gas cap be put in propperly.) I wonder if you filled it up in the two week span and it finally reset itslef after detecting the correct gas cap. If it hasn't come on since then, not to worry. You may want to check to see if any codes are in the computer. As you can see form this thread the older volvos enable a person to go to a little box in the engine to read the codes. The newer ones do not have the little box and it's going to cost money to take it to Volvo. Of course, if it's under warranty, then they shouldn't charge you.

Delgado