There is an extremely important detail left out of these conversations on motion sensor monitors: the care that must be taken when setting them up and adjusting the sensitivity setting knob on the back of the unit (for Angelcare monitors).
Most people treat these as "plug-and-play", leaving them at the factory setting or setting them at the typical recommended setting of 3. In fact, establishing the precisely correct sensitivity setting for each individual setting is crucial. Mattress type, presence of ceiling fans, bedding, baby weight, baby age, etc. - all these require different sensitivity settings. Further, as the baby grows and as changes are made to bedding, the sensitivity must be recalibrated.
I have found the Angelcare motion monitor to be extremely reliable, accurate and trustworthy, and I have no doubt that it could save baby's lives, but When correctly setup and regularly maintained.
Recalibration is done by moving the sensor to it lowest sensitivity and then stepwise gradually increasing the sensitivity until actual breathing movement (combined with room air movement) is detected (no more warning beeps). Then you start at the other end with the highest sensitivity and gradually reduce the sensitivity until you lose the movement detection (the warning beep starts). All the while, of course, you have to keep the monitor from going into full alarm after the warning beep by turning it off, then on again. Eventually, you should end up at a sensitivity level that is somewhere between these two threshold levels. Then you have to sit and wait and watch. In the morning, lift the baby off the mattress and time how long it takes for the alarm to sound. If it's much more than 20 seconds, then your setting is wrong.
If you do all this calibration work and still get apparent false alarms, then you have due cause to ask your pediatrician for an order to get a professional home heart and respiration monitor (with chest sensors) to gather more data and see what's really going on.