In reply to Charlie Wade, I originally asked the question on behalf of my boss (as I was already a FF member, in the new Panda section). He has since replaced the car, but his daughter still uses the Punto. I gather they both got rather adept at watching for all the lights to go off when turning the engine off before taking the key out of the ignition. So long as everything was 'fully off' when they locked the car, it stayed that way. As was mentioned before, a faulty earth or fault in the 'body computer' is most likely.
Pete