you could use this under the bed:
It works like an infrared sensor (same three pins), but it does not need a line of sight. It just reacts to movement as this results in a disturbance in the emitted radio signal. Works very well on my clock
The "high" value when movement is detected is not a full 5V, so I test the Out pin as an analog pin. Anything above 2.5V is an activation due to movement.