If you have some basic DIY skills and knowledge of electrical circuits, a good place to start is purchase of the Volvo wiring diagram for your model.

Mine shows the P window and central locking systems both are grounded at a screw into the floorpan under the passenger seat (LHD cars). So is the power sunroof circuit. Try the sunroof and if that also operates the locks, it almost surely is a poor ground at the above screw. Bad grounds cause a lot of strange electrical gremlins.