Anti Theft Alarm System Suddenly Started Going Off. How Do I Turn It Off?

I've been driving this car for well over a year now and never have i had the alarm go off. Actually the mechanic/collector who sold the car to me said that the alarm was disabled and that it doesn't work. I went to open the trunk one night and about 5 sec after opening it the alarm starts to go off and the lights activate as well like they are supposed to. I then locked and unlocked the trunk, unlocked and locked the driver/passenger doors, and started and turned off the engine...nothing worked. After 3 cycles of audio/lights/audio it stopped. I went to open it the next morning from the driver door after unlocking it and when i opened the door it went off again. I then looked up how to disable it and i unplugged the wires from the Becker box under the passenger front panel. I am now wondering how do I turn it off when it goes off? I would like to have the alarm work properly now that I know I have one. :p Does anyone know how to fix this or is it just better to keep it disabled?