To anyone who does want to disconnect it (say it goes off at the weekend and you can't turn it off)
1. flick the trip switch for the power to the alarm
2. Disconnect the battery - depending where you panel is this, can be a bit of a pain, but it's just two connectors
And yes - my alarm went off when trying to paint around the panel where I set it. Alarm was not set at the time.
Similarly yes it will go off, though sounding rather ill, if the battery goes - there is some beeping in advance of this - last time it happened me, I couldn't work out what the beeping was, kept checking my fire alarm. But then the full alarm went off in the middle of a dinner party (cue working out how to disarm alarm while serving suptious meal simultaneously)