The flush sensor on a toilet faces toward the front of the stall, and the sensor spreads out in a V. It expects to sense 3-4 feet away. It works when it senses a bounce back (which could be anywhere bouncing around the walls as you move).
The flush sensor on a sink is facing down into a curved reflective surface which is constantly bouncing back the signal. It works when it _doesn’t_ sense a bounce back, so you have to block the entire sensor.
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