Ok great ill have a look into that, we arnt sure if there is a siren or just a box outside but it doesnt work atm, we tried shorting the power and battery but it just went off inside! It really does have a life of its own. If theres no siren outside it seems pointless not sure quite what benefit that is might as well just have a fake box on the wall and no alarm!
If I explain how our ADT Galaxy 8 worked it might help you understand what's happening, since my guess is you have exactly the same setup.
We had:-
An alarm panel on the wall inside the garage, with ADT and Galaxy 8 stickers on it.
A standard MK7 keypad at the front door.
Two dummy boxes fitted on the walls outside (which did nothing except act as a visual deterrent).
A loud siren in the lounge (a white plastic box about 10 inches by 4 inches). It also had a loose length of telephone style cable coming from it, which was about the same length as to reach the nearby telephone socket.
An internal speaker, fitted above the front door.
Various door, PIR, and carbon monoxide sensors (a total of 8 zones, hence the 8 in the panel's name).
When the alarm is triggered, it makes lots of noise in the lounge but gives no indication whatsoever outside the building. It's supposed to contact ADT's Alarm Receiving Centre (ARC) via the phone line, and ADT then phone and inform you that your alarm is going off.
Depending on the panel settings, your panel will register a telephone line fault (or the fact that you don't have a current ADT contract) and might not let you reset the fault without the engineers code.
The main alarm panel will be wired into the 240v mains, but will also have a 12v backup battery inside it. Thus if you disconnect the mains then the battery should take over (although it'll probably also log a 'mains fault', which also might trigger a full alarm). Our lounge siren also had a battery inside it (although ours was dead as a dodo), so if I disconnected both the mains and the main panel battery then the battery in the lounge siren probably would've sounded (but ours didn't, probably cos it was dodo'd). This probably explains the chain of events that you describe as "a mind of it's own".
I couldn't agree more that an alarm which doesn't sound outside is as much use as an ashtray on a motorbike, and if our house didn't have an alarm when we moved in it probably still wouldn't have one. However, since it had one, it might as well be one which worked, right?
This led on to a bit of research and, with help from this forum, I installed a new Galaxy G2-12 (12 zone), along with a new external strobe/siren. Most of the panel works by 12v DC so it's not too dangerous if you're confident, and the sensor wiring is a straight swap. Along the way I also discovered that you can get an upgraded version of the MK7 keypad which works with keyprox (a small keyfob you swipe against it to set/unset - very handy when you come home with bags of shopping etc) and has volume control so you can have the button presses quieter (important to us since we come and go at silly times of day/night and don't want to wake each other). I also added the £50 selfmon ethernet module and have full control of the panel from my phone app.
What we now have is a self-monitored alarm costing £1 per month - which texts, emails and calls me (and other people as required) if my alarm goes off, plus it makes noise and flashes outside. I also have all the engineer's codes etc and can amend zones etc to suit. It also has night set (and part set) modes which means I can, for example, disable the upstairs sensor when I'm in bed but still have downstairs alarmed.
Apologies for the long winded post, but I hope it helps you understand what you have
