07 Ford Focus central locking locking itself

I have an 07 Ford Focus Zetec 1.6 Petrol and I am having the problem with the central locking locking the doors without me touching the fob or using the key at all. It happens when I've stopped driving, the engine turned off and then go to open a door from the outside or the boot. If I am inside, the doors will open with no problem.

Can anyone help?