Here is the response from ST in regards to the siren not being stopped when the system is disarmed. I did let them know that the behavior of SHM, and I believe STHM initially, was to stop the siren when the system is disarmed.
Hey there,
Thank you for your patience and efforts.
We looked into your query and I would like to inform you that the Smart Home Monitor will only stop the siren when the intrusion is Dismissed. Disarming cannot stop the siren because disarming the system does not dismiss the active intrusion. SHM will also continue sending intrusion notifications until the intrusion is dismissed, so it’s good to dismiss the intrusion instead of just disarming the system.
I see you want to have the siren stop when disarming the STHM instead of dismissing the intrusion. Unfortunately, there is no feature currently available within the SmartThings app.
I have considered it a feature request and I’ll be making sure to reach out to our development team with that feature request. I don’t know the timeline for the feature update, but the bit of good news I can give you is that the majority of these features are on our radar and we on the support team are looking out for every request to make sure this feedback is heard.
I hope this feature will be added at the earliest.