Every WiFi packet in your home is passing through — and being subtly reshaped by — your body. This post almost certainly walks through the growing field of WiFi sensing: the practice of inferring physical presence, posture, breathing rate, gait, and even keystrokes from Channel State Information (CSI) collected by ordinary 802.11 radios.

The technical foundation is genuinely elegant. A WiFi access point transmits a known preamble across dozens of OFDM subcarriers. The receiver measures amplitude and phase distortion per subcarrier to equalize the channel. That per-subcarrier CSI matrix is, incidentally, a high-dimensional fingerprint of every reflector in the room — walls, furniture, and warm wet sacks of water that happen to be moving around. Feed a sequence of CSI frames into a small CNN or transformer and you get:

• Presence and count — how many people are in a room, through walls
• Vital signs — chest rise from breathing modulates the signal at ~0.2 Hz; heartbeats at ~1 Hz are visible with enough averaging
• Activity recognition — sitting, walking, falling (the "grandma sensor" pitch)
• Gait identification — individuals have distinguishable CSI signatures, meaning WiFi can, in principle, identify who is walking past
• Keystroke inference — finger micro-motions near a laptop alter multipath in measurable ways

The technical audience angle is threefold. First, this is a privacy story hiding in plain sight: the 802.11bf amendment (ratified to standardize sensing) means your next router may ship WiFi sensing as a feature, and there is no equivalent of a camera LED to tell you it's on. Second, it's a signal-processing playground — CSI extraction used to require patched Intel 5300 firmware, but the Nexmon and ESP32-CSI toolchains have democratized it. You can build a through-wall breathing monitor for under $20. Third, it reframes threat models: an attacker who compromises your neighbor's router may not need to see your screen to know when you're home, when you're asleep, and roughly what you're typing.

Most engineers still think of WiFi as a data pipe. It's also a radar, a biometric sensor, and — increasingly — a surveillance surface baked into every building. Worth the ten minutes.