The linked paper "Et Tu Alexa?" uses a single smartphone positioned outside of the property
> We show that just by sniffing existing WiFi signals, an
adversary can accurately detect and track movements of users
inside a building. This is made possible by our new signal model
that links together human motion near WiFi transmitters and
variance of multipath signal propagation seen by the attacker
sniffer outside of the property.
> We show that just by sniffing existing WiFi signals, an adversary can accurately detect and track movements of users inside a building. This is made possible by our new signal model that links together human motion near WiFi transmitters and variance of multipath signal propagation seen by the attacker sniffer outside of the property.