Couldn't find the exact one I ordered a few months ago, but this type of pressure sensor aliexpress link is apparently a common bed occupancy sensor type used when building medical beds for hospitals and care homes so staff can be alerted when a patient gets out of bed. I'm not sure the specific model I linked is quite right specs-wise, but the idea is that it goes on a slat or other surface between the matress and the frame that doesn't necessarily get the full weight of the occupant, but still gets enough to measure on the sensor.