Strange locking issue

I've got a strange problem with the locking on my 2000 ST200.. Everything works normally with the system in that the remote locks and unlocks etc but if I just unlock the drivers door and then get in and drive off, after a random period of time the rest of the doors unlock by themselves! I've heard the locking mechanism going a few times during a journey as though it's locking and unlocking.. Not a massive problem but worried it may cause problems.. Can anyone advise what may be causing this??
Thanks