It may be a silly question… but is Smart Home Monitor a local execution qualified SmartApp?
If so, then if the intrusion sensors (contact, motion) and the siren are also qualified for local execution, then the alarm will likely be able to sound when the SmartThings cloud is down.
The problem: How do you shut it off.
Suggestion: Are Minimotes also local qualified devices? Hide one somewhere and have a button press tied to disabling the alarm? Hmmm… but what SmartApp can do that? It also must be qualified to run with local execution!
See the problem?