Stray RF (Radio signals) from a variety of sources can trigger that module.
Do the locks trigger only when the car is running? If so you might have excessive hash from the alternator, maybe a bad regulator or arcing brushes.
Alternators are famous for broadand hash/noise. It can be a nearby Transmitter
Fire/'police/taxi/ham/CB/commercial broadcast/or industrial RF stuff. See if a portable shortwave radio goes crazy with noise from your car.

Central locking in the 850 varies between 4 and 5 door and some have an additional deadlock set relay, Thus you need to find the right schematic for
your specific model. Even with the schematic, there's no theory of operation
included, so it requires some guesswork.
Your issue will require the John Wayne approach where you try different
Central locking components, since it's intermittent.
You might visit a junkyard and get a new central lock relay module which has
the control module and the high current switching for the lock motors.
That would be my first candidate for exchanging.

There's a fuse in the 850, F32 that runs the radio remote locking & alarm.
You might disable that and see if the locks still trigger on their own, but as you pointed out, it only acts up every third rainy Tuesday. :-(

It might be easier to pick a lottery ticket than to say for sure what's wrong
there.

Bill