Hi Andrew, when I worked as a Customer Services Manager within Ford UK, there was an issue with Explorers unlocking themselves if they were parked near mobile masts, Police Stations and supermarkets - all areas with concentrated radio/electrical activity.
It is very un-nerving just the same, as your confidence in the car is adversely affected.
What I would suggest is to keep a log of when this occurs, and build-up a history of the frequency (i.e. every time you pass by, or only at certain times, etc). Then, I would change my route, and not go near that junction at all - see if the occurence alters or ceases entirely. What you are doing, is problem-solving at its most basic - treat the junction as a constant, and everything else (the car, driving conditions, time of the day/week) as a variable. Hopefully, the car should not display this rogue behaviour when its away from this junction - so you can rule out any faults with the car. If its just as bad, then something is untoward with the car (but at least you have an ability to replicate the fault with the dealer).
Also, if you have built-up a pattern, you can contact your council, who can monitor for the useage of equipment that is causing this behaviour.
Hope that helps.