Have a Clifford Concept 50 on our FRST. Since returning to the car at Tesco's we have a problem.

Everytime I arm the alarm the siren goes off straight away If I leave it to passive arm it is fine Should I take this as an omen to ditch the shonky Clifford or does anybody have suggestions as to the cause. It is pissing with rain if that may have anything to do with it