Volvo 850 unlocks itself

I have a 1997 volvo 850 sedan in good health. Over the last week, I have returned to my car to descover that all of the door locks have mysteriously unlocked themselves. This has happened about a half a dozen times thus far. I park in the ally behind my home, in the city of Baltimore, MD. The only consequence has been the theft of the ash tray and the $5 in change that I use in parking meters. Can anybody tell me why this is happening, before my car gets stolen.

Perhaps there is a door lock relay heating up during the course of daily use and when cooling down after the vehicle is shut off, is tripping a circuit.

Is it some ignition system related fault?

Techs, is that too far fetched?

No one has attempted to answer this question, that’s why I’m shootin’ in the dark.