Alarm on and wont start

Heard a car alarm going off earlier today and it wasn't as loud as mine but went out to investigate anyway. Sound similar to my volvo but like battery dying and my hazards weren't flashing but only one other car outside. Tried to open my car and disable alarm with fob and spare fob but nothing. Alarm went off only when drivers door open. Car wouldn't start battery totally flat. Battery now on charge and see what happens later on. Any idea's as to why alarm would suddenly start sounding and not a full volume, is this something that happens why battery is almost dead.

What car do you drive, mine is a V70 T5 and my alarm went off the other morning about 3am, the hazard lights flashed and the alarm sounded, in the morning the battery was okay, but the day before the battery was dead because of the car hadn't been driven in over a week, and that nights air frost finished it off. So the battery was fine, but I found the bonnet was not locked on the drivers side after me having the bonnet open the previous night, and have put it down to that moving and setting off the alarm. The strange thing also, was in the morning, the red flashing dash light was not flashing twice the speed too let you know the alarm has been activated. I had an issue a few months back with the alarm going off whenever it felt like it, didn't find out the cause in the end. It could be the drop in voltage that set it off?? How is it now you have charged your battery? Andy..

We had a similar problem with an 850 estate alarm recently, had new batteries installed in the key fobs and both fobs re-programmed to the car, we have not had any problems since. Our local dealer only charged for the batteries (Claytons of Mansfield)

Thanks all after charging the battery it has been fine since. I was told by a the guy I got the car off that because of the cold weather here lately (-10oC the night before trouble) that the drop in volts sets the alarm off because it thinks its being tampered with or something along those lines. Been fine since. He also said that all the sort journey's with heaters etc on wont have helped. I really should put it in garage of a night lol. New battery is on this years list along with new pads and discs etc. I have a 1995 850s 2.0ltr 10v...T5-R replica only missing turbo engine (all work done by a previous owner)...I'll upload a few pics.