hello,
i think i know exactly what is wrong with your car,this is very common on later model volvos and was a problem with the door locks due to an inferior microswitch inside the locks themselves. to repair the problem the lock needs to be replaced,this is a current volvo service campaign and if your vehicle is still under warrenty your dealer will replace this lock for free.if it is under warrenty my advice would be to say that you have had problems with ALL the locks on various times and then they will replace them all.otherwise each lock usually fails in turn one after the other until all four end up being replaced.if no warrenty then you just have to bear the price and have it done.
hope this helps.

steve.