2002 V70 T5

I've looked through various threads about central locking on here. Sounds like it might not be the most dependable system in the world?
Anyway, the issue with mine is that it keeps re-locking it's self after I've unlocked it.

I've had cars in the past that would re-lock themselves if they had been unlocked and left untouched. Makes sense. But the Volvo will re-lock it's self unless I turn the key in the ignition.
No matter which doors I open. In fact, it even re-locks it's self with all the doors wide open unless I turn the key in the ignition. So far this has only presented a minor irritation, but the day is coming that I or my better half will lock the key in the car. We all know it is.

So, any ideas? Can the system be somehow reset? Could there be a faulty switch / sensor somewhere that I don't know about? The interior light comes on/off as expected, so I assume all the door switches work, although I can't say that I've actually tested these.

Any help appreciated.