Mine used to go off silently all the time, when I first got it. Turned out one of the cables to the rear of the siren had dissolved so whilst I'd get a visual flash of the indicators to say it was armed/disarmed I got no noise from it all and I'd always jump in the car thinking it was disarmed only for the indicators to start frantically flashing away as I was groping around trying to get to the remote control to disarm it. I fixed the cable to restore power to the siren and therefore sound but found the best thing was to never bother arming it in the first place.
If you had a flat car battery, recharging it or replacing it will have reset and re-armed the alarm so you'll need to disarm it with the remote.
It's possibly also the alarm control unit that's fuggered. That's the black box inside the dash void, roughly in front of the passenger seat. Not sure whether just unplugging that altogether would solve the problem or whether that might introduce any other electrical weirdness with the lights (but luckily it wont immobilise the car in our case); easy to try though.