No, this isn't some experimental concept gadget, but rather a real handheld device that lets you see through walls using Wi-Fi signals. MIT researchers have made a portable device that you point at walls to see whether people are moving closer or farther away from the wall. It works by emitting Wi-Fi signals at a wall and uses their reflection off people on the other side to determine movement. Continue reading for a video and more information.
The Wi-Vi array emits two types of signals that are the inverse of one another. If both signals hit a still object, such as any furniture or the wall itself, they cancel each other out. If they hit a moving object, however, the canceling effect doesn't occur and the moving things show up in the Wi-Vi.
Once Wi-Vi finds a moving person, it's able to track that person easily while ignoring all the still stuff around him. It's also able to track more than one person at once. In their paper, Katabi and Adib showed Wi-Vi's distinct output for three people.