(1) Moving some object toward or away from the device sets off the alarm light.
(2) Moving an object across the beam does not cause a Doppler shift, and therefore does not set off the alarm.
(3) The frequency of the emitted wave is about 40 kHz. With a wave generator and supertweeter, one can shine waves of about 40 kHz on the receiver transducer, fooling the device into thinking that a moving object is in the vicinity (as seen in the photograph).