Ford focus central locking ghost?

Hi, I have a MK1 focus face lift 5dr hatchback 2002. Central locking works perfectly, however, after a random distance the locks make a sound like they've been activated but don't actually operate. They only do it once but nearly every journey is affected. It's very similar to the automatic locking found on some makes at a certain MPH. My model is not fitted with this option.
Bearing in mind this problem has only recently appeared. I was wondering if anyone had come across this phenomena before.