The first step in the researchers’ approach is to locate the Wi-Fi transmitter. They do this by measuring the change in the signal strength as they walk around outside the target building or room. Indeed, they have created an app that uses the smartphone’s built-in accelerometers to record this movement and then analyzes the change in signal strength as they move. In that way, it is possible to number-crunch the position of the transmitter, even in the presence of numerous reflections and distortions.
The researchers say that by walking back and forth a few times outside a room or building, they can reliably locate the transmitter. “We found that consistency check across 4 rounds of measurements is sufficient to achieve room level localization of 92.6% accuracy on average,” they say.
Having done that, it’s just a question of waiting. Provided nothing moves inside the target building, the Wi-Fi signal will be constant. But any small movement changes the signal in a way that is straightforward to measure... It’s not hard to imagine how a malicious actor might use this to work out if a building was occupied or empty.
https://www.technologyreview.com/s/612375/using-wi-fi-to-see-behind-closed-doors-is-easier-than-anyone-thought/
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