I stumbled across a video where a security researcher walked through how Wi-Fi routers can be turned into passive surveillance tools, capable of detecting people moving inside a room without any camera, microphone, or wearable on the person being tracked. A follow-up search led me to a project that demonstrated the same effect on a hobby-grade ESP32 setup. The claim was striking enough that I wanted to see how well it actually held up when I tried to build it myself, on my hardware and network.

The mechanism, once you look at it, is really simple. A human body is roughly 60% water, and water absorbs and reflects 2.4 GHz radio waves rather effectively, which happens to be exactly the band most consumer Wi-Fi operates in. Every Wi-Fi frame the router transmits reaches the receiver over multiple paths at once: a direct line-of-sight path and several reflected paths bouncing off walls, furniture, and bodies in the room. When someone moves through that space, the geometry of those pat...