It’s a sci-fi trope that you can ‘scan for life signs’ and detect if there are humans — or suspiciously human-shaped aliens — present, but in real life it’s harder than that. [The Masked Bear]’s wifisense-pi project isn’t really scanning for signs of life, either, unless you happen to consider breathing a sign of life. Even then, it’s not detecting breathing per se, but the subtle motion that goes with it: it’s a very sensitive motion detector that relies on the fact that we fleshy bags of goo disturb WiFi signals with our presence, and motion alters those disturbances.We’d probably waste a lot of time watching the signal graphs on the WifiSense-Pi dashboard.The device uses an ESP32-S3 to measure the radio channel 100 times per second, while a Raspberry Pi 4 provides the signal processing muscle. It can detect the slightest motions, and even determine the presence of a perfectly still human by their breathing, though you can hide your presence for as long as you can hold your breath. A single sensor, no matter how sensitive, cannot give position information, and while multiple humans will distort WiFi more than a single one, [The Masked Bear] reports you cannot reliably extract that signal. So this project answers the question: “are there humans in this room?” Or, even more likely, “are there any large breathing animals in this room?” We can’t imagine a 50 kg Mastiff looking any different to this sensor than an equivalent mass of quivering human flesh.Before you dismiss this as just another motion sensor, keep in mind that it is sniffing the signals already present on the 2.4 GHz band, and, like the WiFi signals themselves, it can work through walls. So we think it’s pretty nifty. Of course, there are many other ways to detect humans, from machine-learning cameras to millimeter-wave sensors to a simple PIR. This isn’t the first project we’ve seen that uses WiFi like this. It isn’t even the first with an ESP32, but it’s an interesting implementation worth checking out.