The system is still running on battery backup,there's a battery within the panel and the outside sounder.
Now you have isolated the mains you can remove the panels cover and disconnect the battery.
The outside sounder will run until the battery drains,by the time you read this the alarm has most likely silenced anyway.That's the easy bit out of the way.
sorry I am going to pass on answering the rest of your question regarding recommissioning,maybe one of the guys can offer better advice.
no name
April 2012