Alarm going off

[Phil78]

Hi all, put the front end off my 96 gti on the ramps today to fit a cat as its due an mot and after around 15mins the alarm went off, this has never happened before, ive never had an alarm without a fob before so unclipped the battery as its all i could think to switch it off, i fitted the cat and put the car back down off the ramps and refiited the battery and it started again, ive had to take the siren off to shut it up lol, have i got a fault somewhere or do they need reset or im not sure what to do with it. Thankyou