This is usually one of two lamps in the rear tail light. In the evening it will be most evident.

Rob

These are the daytime running lights. There are around 10 of them either on the sides or in the taillight housings. This one is probably in the rear somewhere and has burned out. I lost one after about 40,000 miles and the dealership replaced all ten under warranty. Their theory was that when one goes, the rest will soon follow. A couple of months later a headlight bulb went out. The dealer replaced both under warranty under the same theory.

They told me that running all these lights day or night tends to burn out the bulbs. Like I said, it took about 40,000 miles for me in my 2003 S60.