Ford mondeo alarm activates by it self.

Hi! recently i bought a frod mondeo 55reg tdci, after a week i found out the car has a problem with the alarm, which sets off by it self, normally when i do short journey ( 1 mile or less ) it set off sooner & when i drive more than 5miles some time it doesn't go off for days, i,m having sleepless nights, bescause my bed room face toward the drive way, which wakes me up two or three times a night & like i said some time it deos not goes off for days. can you please help? Thanks