ST does have very rare ghost events where things randomly turn on, but that’s a bug in the hub / cloud sending the commands, not the devices themselves randomly turning on. It’s never happened to my siren though.
I do have a failsafe rule though, set to turn off the siren after 10 minutes of staying on… saves the neighbours sanity should there be a false alarm.
As you are using a custom handler, the siren is entirely reliant on the cloud, so a SHM rule to turn it on won’t fire without internet connectivity and power to the hub.
If the siren gets triggered by a rule before the internet / power fails, then you won’t be able to turn it off, short of physically sliding it off it’s cradle and pulling the battery.