Have had exactly this myself on a Ford, (locking and unlocking at random as you go along) and would be fairly confident that it is the solenoid in the door locking mechanism.

The door lock works on the basis of an electrical circuit with gap in it. The solenoid in the circuit has two contacts with a small space between them, which gets pushed together, or pulled apart as you turn the key (or activate the mechanism via the fob) thus closing or breaking the circuit, and working the lock. Because it's such a fine tolerance, when the solenoid gets worn, then bumps, temperature changes or moisture can all cause the contacts to momentarily touch together and fire the lock.

Best solution is to change the whole sealed locking mechanism (sometimes referred to as the Actuator). As far as I recall, on my Fiesta the part cost around £70, but I didn't pay for it as it was in the dealer's second-hand warranty period. It's probably not really a job to attempt yourself unless you are reasonably confident with stooging around inside the door panel, and with electrics. That said I am told it's not too difficult if you are actually a bit handy.

The other thing is, that there is a mechanism on each door and it's not easy to be sure which one is the culprit. Rule of thumb is to go for the drivers door first as this usually gets most wear. (You may be able to tell by listening, and watching the door levers inside when it malfunctions - Which one seems to start clicking first?)

It's a pain, but you ought to get it sorted, otherwise the security of your car is compromised if it unlocks when parked up somewhere.

Hope this helps. RL

It's called bouncing' I had it on my mk2 Mondeo, usually the microswitch.

As stated it will fire off at random whilst driving.

Disaster struck at a petrol station, I refuelled and went to pay, on walking back to the car I heard the car lock itself, and of course the keys were in the ignition -it was early Sunday morning- normally I would remove them. All windows were closed.

Fortunately, there was a taxi on the pump in front, I lived 5 mins away and was back within 10 mins with the spare key.

I would suggest disabling locking by removing fuse, and locking manually with key until fixed.