2002 Ford Mondeo central locking

Hello All can anyone help me..The central locking in My ford Mondeo keep's locking by itself when the ignition is switched on and even when i'm driving along it justs locks out of the Blue..I found this out the hard way one morning at 4am when I was going to work..I took my dog down to do her bit's and bob's and started the car to warm it up went back 5 mins later only to find the car locked withthe keys in the ignition and the car still running..to cut a long story short I had to break a window to get in as I couldn't wait 2 hrs for the AA to come and I had to get to work..any ideas

Thanks..

Peter