There was a big debate about this some months ago. A poster averred that you could lose connection with one of the sensors and not know it for a considerable length of time, as it would not necessarily set off the alarm. He was pooh-poohed by a lot of people, including me, saying that couldn't happen. It seems as though he was closer to the truth than we were.
The whole thing makes me concerned about the actual reliability of the system. I may have to do the check with removing a sensor, and make sure the system is working as it's supposed to.