Gait recognition! I wonder if they can remotely decode passwords being typed on physical keyboards, since keyboards have a well-known spatial layout.

The characteristic length of 2.4GHz waves is ~12cm. This is roughly their feature resolution size. But you can do better with more antennas, more frequencies, more time, and phase information.

I've tried this [0] "typing biometrics authentication" out in some test apps. It worked well in detecting typing a password between two hands, just my left hand and just my right hand. I tried to do the same cadence - but it seems to also notice keypress duration as well. Not sure how I feel about my typing rhythm being a personal signature.

[0] https://www.typingdna.com/

Search-as-you-type interfaces, including search engines, often have enough signal to perform keystroke timing for the same purpose.