Police could set up transceivers outside a building and compare spectrograms of suspects walking vs. crime scene footage.

"Yasamin Mostofi asks us to imagine this scenario: police have video footage of a robbery. They suspect that one of the robbers is hiding in a house nearby.

Can a pair of off-the-shelf Wi-Fi transceivers, located outside the house, look through the walls to see who’s inside?

That’s easy to answer, since we’ve seen it done before."