I'm the second owner on my 2002 S6. There are three antennae on the car (see photo). The tall one is easy; OEM antenna for radio and navigation. The big "blob" antenna with the tape-wire is for the after-market XM satellite tuner the previous owner installed.

But I'm stumped as to what the 3rd antenna (kinda looks like a thumb sticking up) is for. I know it is after-market. Does anyone know the purpose of this antenna?

I've seen some higher-end installed car phones have a similar antenna like that. Considering the year it might be possible. I don't know if your car has OnStar or not but it could be that as well.

No OnStar, and there is no evidence of a former after-market phone installation...

After market alarm or remote starter?