I have ahd this volvo from new and once or twice in the past the alarm has gone off for no reason (more often in the rain).

Just recently it has taken to intermittently going off in the middle of the night in any weather. nothing external seems to be setting it off. sometimes using the fob it can be reset and that is is, other times I have to key lock the door to disable the alarm.

Anyone had a similar fault or any ides as to what the cause is likely to be?

One of the door locks can be going bad.
The dealer should be able to read out what it setting the alarm off.

Yeh same problem , it was the sensor under bonnet , I just lock with key , no hassle

alarm backup battery on siren may at it's end of life.. s40 I don't think u can read out alarm cause. P1 and P2 chassis is the one that I know u can read out alarm cause.