So recently I noticed that the alarm seems to be randomly going off on its own. However, there is no indication that this is happening: no sidelights flashing, no blinking of the small led light and in general absolutely no indication that this ever happened!!!

So, what's wrong? Can it be that the alarm has a backup battery that needs replacement? Some other fault? Any ideas?

I found the manual and it says that the alarm model is M7199us/M8199us if that helps

It turns out that the siren does have backup batteries and I decided to replace them as that seemed like a probable cause of failure. To my disappointment this didn't seem to fix anything as the siren still doesn't sound when triggering the alarm.
So, clueless once again as I'm not sure if it's just the siren or the whole system! It would be interesting to try this on another Grande with the same alarm system, but I don't know anyone around who has one :idea:

these people made the alarm as an aftermarket option
http://www.metasystem.it/index.php/msy_ita_en

I have in the past seen a manual for the M7199 online but cant find it anymore, I guess if you email them they may provide you with a copy.

I have the same issue as you and just lock the car with the key to disable the alarm fully.

would be nice to know the solution though

here are the limited instructions - the number of beeps when you disarm the car might help you troubleshoot

In my case the siren started going off randomly and for a few seconds only (random also) without any other indication e.g. no alarms/indicators flashing, no LED light blinking and no beeps when unlocking the car. In one case I noticed that unlocking the car (and hence disabling the alarm) wasn't stopping the siren!
During this initial period, the siren would still sound, when actually triggering the alarm by opening a door with the alarm armed etc.

This led me to believe that the issue was with the backup batteries in the siren (it has two CR123A batteries) which I have now replaced. After replacing them I haven't had an incident of the siren going off on its own but it also doesn't sound when I actually trigger the alarm (everything else seems to be working normally). Also there are no beeps (when disarming from a triggered alarm) as I suppose these also come from the siren. I've checked the power supply to the alarm and fuses and they seem alright.

So it feels like it must be just the siren that is not working for some reason and I've found one online http://abacuscaralarms.co.uk/alarmshop/index.php?act=viewProd&productId=462
However, I'm a bit hesitant about getting a new siren without making sure that it's the only problem. Unfortunately, it seems like the only way of making sure is by testing it on another car or testing with a siren that we know already works

Worth mentioning that the problem was that the alarm wiring (external fuse box) had gone loose

Hi, I must say I'm really impressed by this update !! It just took a bit more than two years to get published …
Anyway, thanks for letting us know

BRs, Bernie