Detecting Online/Offline Devices & Creating Rule

[HAL9000]

There are user-written apps than can report on devices that should report in periodically (e.g., battery reports) but have not. Have a look here:

Is there a definitive and reliable way to actively monitor devices going down?

This is most essential with security related application. I've been deploying 433Mhz alarm panels for many years and the issue is not being able to catch the sensors going down (broken or battery issue) right away which poses a security concern. Thus, I need a way to make sure that I am alerted if a sensor goes offline (specially door, motion, and other sensors). The alerting part can be a sound or TTS speak command, or whatever works. But it's the monitoring part that seems to be the challenge…