By chance, Andreas Spiess did a brief review of the new Hi-Link LD2410 24Ghz radar sensors, which are designed for presence detection.
They look promising and aren't expensive, so I bought a few along with the dedicated UART breakout board made for them.
My little bit of testing suggests they're really quite good at detecting people within a few metres, whether they are moving or not and people are rarely completely still. At close range it can tell if you're breathing.
While you can just connect one of the pins to a GPIO to signal presence they use a serial protocol for configuration and more detailed readings. However as they are comparatively new devices there wasn't a library for this.
Between reading the work somebody has started on integrating them into ESPHome and the slightly confusing manufacturers datasheet I've cooked up an Arduino Library and submitted it to the Library Manager. I hope people find it useful.
Now I need to make some breakouts for them, they use 1.27mm headers, and attach them to my rover.