I'm assuming it is a PIR sensor in a room that is tripping the alarm - if it is, and animals can be ruled out - most likely cause is the Sensor is faulty and needs replaced - they are not expensive About �10 - �15 will buy you a good one ( I use ones manufactured by Honeywell and have found them to be excellent )
Or if you want the bees knees of detectors, a Dual tec Sensor which uses harmless microwaves as well as PIR.
They are wasy to replace - and you can DIY six wires Positive, Negative,Tamper x 2 NO and NC.
Once you have the cover removed the alarm will trigger if the tamper circuit is wired into it, just dont allow the positve supply wire to touch any of the other cables or you will blow fuses on the alarm motherboard.