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Same reason an iphone will not respond to any input with a fingernail or pen; but will to a nose, finger, or other sheepy appendage that is fleshy
My money is on capacitance (rather than IR) ... http://pcbheaven.com
... The sheepy appendage at a distance of a couple of inches would not present much capacitance to ground. If it was sensitive enough to detect the difference it would tend to get confused easily by electrical interference and humidity.
A capacitance switch needs only one electrode to function. The electrode can be placed behind a non-conductive panel such as wood, glass, or plastic. The switch works using body capacitance ... When a person touches it, it increases the capacitance and triggers the switch ... These devices can also be used as a short-range proximity sensor.
Maybe it's a "heat seeking" button. What happens if you are wearing gloves (other than gettting warm hands)?
Or...you could dangle an ice cube from a string. See if it likes that.It could sense movement although I don't see an obvious aperture. Perhaps sonar. I've seen similar wireless badge readers, but that is a whole different device.
It could be similar to this http://www.smarthome.com/77410/Enforcer-Single-Gang-No-Touch-Request-to-Exit-Sensor-Switch/p.aspx
Here you go!http://www.jwsltd.co.uk/touch-to-exit-touch-switch-178-p.asp(Don't waste your time downloading the data sheet.)