I think the techcrunch article (linked on one of the comments) mentioned that if they can't get a gps/wifi signal they look at lighting (camera) and sound (mic)

I think the techcrunch article (linked on one of the comments) mentioned that if they can't get a gps/wifi signal they look at lighting (camera) and sound (mic) to determine if people are in the same location. Neat use of the technology but - yes, a bit creepy.