Scantronic 9448 tamper alarm keeps going off?

Hi,

My alarm has been working well for years and all of a sudden the tamper alarm has been going off when the alarm hasn't been set (but also happened when it was set) e.g

1. I'm in the house (alarm not set) and the alarm will sound and the tamper light will be on.

2. We are asleep and the alarm has been set (for a few hours)and the alarm goes off.

I have no idea what causing this, we haven't touched the sensors etc

Any idea's

Steve