All that diatribe and you leave out one of the most common reasons that causes an SAB to sound, never mind, we can't all know everything.A warning of some sort that does not indicate at the control panel?It will be a tamper from your siren most likely.
Check that all the original mountings are still secure.
If the backplate moves then the rear tamper microswitch can activate.
Oh boy, you need a decent system
I wonder if the (help?) is in the manual?
Well just consider this - when you fit the alarm you know there is a spring at the back of the backplate because you can see it and you can hear it because as you screw the screws into the wall you can hear the microswitch click. You will also have seen reference to it in the manual.
ps
Bert you don't even know what an SAB is.
It would not be unusual to forget this two years later if you had only fitted one system.
However as you say your professional systems alert you to everything in the panel to save you using your noggins and common sense.
Five things are capable of setting off the siren.
1. An activation signal from the panel
2. A signal from a sensor if the siren is in master mode and the system armed or a panic signal
3. A signal from an activated linked smoke detector
4. Movement of the siren lid or base allowing a tamper fault
5. A local RF blocking.
So using the simple process of elimination we can see that according to the OP it won't be 1 , 2 or 3 so we look at 4 next..
simples and you don't need a panel reminder to work through that simple list.
Perhaps it's the difference between a Pro Alarm Installer & a DiY alarm installer?