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Humans can be tracked with unique 'fingerprint' based on how their bodies block Wi-Fi signals
(www.theregister.com)
This is a most excellent place for technology news and articles.
They explicitly went into the advantages over cameras:
So perhaps a building takes a picture of everyone as they come in the front door and also establishes a 'WhoFi' profile for that person. They could keep track of their movement through the building while maintaining an actionable correlation to a photo.