Stanford has a group called "VAIL" which is an acronym for the Volkswagen Automotive Innovation Lab. [1] As you probably know, Audi is a subsidiary of the Volkswagen Auto Group. AFAIK, this group was borne out of the DARPA Autonomous Car challenge, where the Stanford team partnered with VW to win the challenge. [2]

They've produced several different VAG-based models including several Audis that mainly focus on self-driving and similar AI. One of their more high profile recent examples was an Audi TT that climbed Pikes Peak without any human input. [3]

Some pretty cool stuff, and likely the cars you saw.

[1] http://www.stanford.edu/group/vail/ [2] http://en.wikipedia.org/wiki/DARPA_Grand_Challenge [3] http://www.youtube.com/watch?feature=player_detailpage&v=Y88...

Tesla HQ is near Stanford, which has some Audi cars which have sensors on the roof.