@bus_factor Some of the #mmWave (Radar) Presence sensors may be able to detect bed presence through the mattress, mounted under frame. I have not tried this yet but I’ve been planning to. The Aqara FP-1 or Aqara FP-2 are candidates, but maybe too pricey for an experiment. The Apollo MTR-1 is far less expensive and excellent for short range. It is also small and with a sturdy enclosure you could easily place it between and mattress. These sensors can pick up very small motions, even the human heartbeat (sometimes through walls). I use both Aqara (easier set up) and Apollo MTR-1 for room and zone presence detection, with #HomeAssistant for integrated automation.

I believe that's how most people do it. It works for me. You can also put two in parallel or just add a resistor across the leads to divide the resistance to make it more sensitive. If you used a potentiometer it would make it easier to adjust to hit the exact threshold.