One of your taillights burned out. There are two on each side, check them all. If one is burned out, the others will soon as well, It's a good idea to purchase four and replace them all at once. All four of mine burned out within five weeks of each other. It's best just to spend a few bucks and replace them all. You'll save yourself trouble later and a potential safety hazard. You'll need to remove the lower bulb holder to get to them.

This happens a lot as the daytime running lamps on the S80 aren't DRL's in the traditional sense of low beam headlights or an even smaller bulb is always on. On an S80, the complete lighting system in on, just as it would be if the switch was turned on. This means the taillights are on whenever the car is running.