Help with a Concept System?

Can anyone advise me what might be the problem with my Concept alarm system.At night I set the system omitting the upstairs zone.In the morning when I enter downstairs sometimes the alarm beeps and I am able to switch the alarm off as normal and other mornings there is no beeping sound and within seconds the alarm has gone off.It is about 15 years old but was serviced (after my husband cut a wire accidently) a year and half a go.I really can't afford an engineer to come out and my husband is away so I was wondering if it was something simple! Many thanks.