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Would be interesting for a home security system, or for enhancing Nest so the users don't have to walk past it for it to know you are in the home.

Also the paper notes this approach has fewer privacy concerns than other tracking systems, which is true to the degree your goal is tracking locations in a house. However if all the sudden we all ended up with tracking systems in our house that would be a privacy concern.

Useful for detecting if aged or ailing family members haven't moved in a while, and check on them.

A team from MIT has created a product for this specific purpose using a very similar technology: http://www.emeraldforhome.com/

Not using WiFi, but their own RF spectrum. Very cool though.

True, some of these wifi tracking systems can track heart beat as well.

Better home automation, perhaps.