Why does my Optima XM alarm's internal bell keep going off?

Wondered if anyone could advise. Last year my Optima XM alarm started playing up. The internal alarm kept going off. The alarm was not set ( I was often in when it happened), the external alarm didn't go off. When entering my code it would stop and the "Attack" light would be highlighted.

I called an Engineer who replaced the battery and charged me £100 for the priveledge. This was just under a year ago this month.

Anyway, the internal alarm is playing up again. It has gone off twice tonight, and it's not the first time. I tried turning it off at the mains, but although the power light goes off, the DAY light is still on, which I presume means the battery is OK.

Does anyone have any ideas why the internal alarm thinks it's being attacked. I am loathe to call out the engineer again because a) it was expensive and b) doesn't seem to have fixed the problem for very long.

Any advice would be much appreciated.