Microphone could be one possiility, what about additional knowledge from GPS (knowing you're at a location people have checked in on Facebook as being at the op

Microphone could be one possiility, what about additional knowledge from GPS (knowing you're at a location people have checked in on Facebook as being at the optometrist/glasses store, as well as being in locations where people post about such things without checking in. With as many different users doing as many different actions as Facebook can see, it wouldnt take much to piece it together.