I have a Chubb house alarm that we never use, however as part of the alarm are wired smoke alarms.  For this reason (and resale) I am reluctant to pull it out.  Normally having it is no problem however when the power goes out it starts wailing in the outside siren.  Its not its normal powered wail, its more like a half effort.  Nothing I can do will turn it off, the number pad etc. has no power so I cant put in our number to stop it. 

I have on one occasion had to come home to try stop it after hours of wailing.  I tried everything to stop it.  Got up in the roof and found the alarm box, tried to pull the siren cables, tried to remove the small auxiliary battery.  Nothing worked, thankfully the power came back on.  However I need to make sure that never happens again.  Imagine a  3am power cut.  The neighbours would not be happy.

So the questions for any experts/enthusiasts/knowledge holders

1)      Why does it wail even without power on (mains off) and the auxiallry battery removed?

2)      Whats the best way to deal with this, short of cutting the horn wires outside?

I can take a photo of the box inside the roof if need be.

Cheers