I removed mains ( total rewire of house ) and the panel on its standby battery sounded to say no power, reset it to silence it and then some time later it repeated the no power alarm so I dis-connected the panel's battery which removed the "all OK" 12 volt to the siren which went off on it's own internal battery until that too was disconnected,In your scenario are you saying the control panel was dead and that the internal battery was also dead?
Because there is no 12 volts to send the siren telling it NOT to sound it will sound. This ensures the siren will sound if the wire to it is cut in the hope of silencing it.It would already have activated when the power and internal battery had gone dead. This is the usual scenario when there is a power cut and everyones alarm activates because the internal battery hasn't been replaced.
How does a Yale system get the siren to sound if the control panel has no power to send a transmission to the siren telling it the panel is no longer able to function.