after it has gone off. disarm the alarm put the key in the ignition and turn it to point 2. The alarm led will flash a number of times tellng you which sensor set the alarm off.
Then look in the alarm handbook and match the flash sequence in the grid and that will tell you your problem
see theres a person who bought a shit hot alarm and read the instructions
when my goes off i lie in bed thinking "thank fook i can still hear it, means its not been nicked
, when it goes quiet i look out the window while getting dressed