97 Avenger ES

Ok here's the problem, as soon as the driver door is opened the alarm starts going off. We bought the car used so we don't have a remote to shut it off. I will be buying one soon but what would make the alarm go off when you open the door? The car was not locked prior to opening so the system should not have been armed. I just read about turning the key in the pass door lock but the key in the ignition did not shut the alarm off. The alarm just kept going until the time ran out (I assume it only goes off for so long). Thanks for your help