Optima Compact keeps going off?

Having set the alarm, it appears to react as normal. However, after approximately 1 hour, the alarm bell alert is activated and sounds. We have checked to ensure there is nothing that can be moving in the house i.e. drafts etc. No explanation. This has now occurred during eveings and daytime and we are 100% certain it is not because someone is trying to gain access to our house. It must be the system itself, but we cannot understand why it would become activated?