There is someone on Reddit who is tracking all the unique IDs of every wifi-using device in their vicinity. And because many (or even most) people now carry wifi-enabled smartphones...
This person wrote a program that recognized and filtered out all of the static devices from the neighbors, judged how far away someone's device was based on the signal strength, and tagged certain IDs as belonging to certain people/groups. The code for this project has been put on Github, and it is not difficult to do with current technology.
They are using this to sniff out, say, suspicious loitering. Or emergency vehicles. Or pushy missionaries.
In a few years this may well be standard equipment for many businesses (to identify repeat customers, and to know if people they've banned from their establishment are trying to visit). It could be used by wealthier people and suburban subdivisions to identify strangers in their neighborhood (to "gently escort" them out of the area). Someone else will certainly come up with more possible applications.
Unlike cellphone data, wifi can be sniffed with extremely cheap equipment - in this case, an antenna attached to a Raspberry Pi. That's going to make sousveillance a lot easier.