I have just purchased a new Mazda 6 estate, and have the same problem of finding the car has not locked itself, the first few times I never noticed till I came back to the car which is a bit of a problem, but as I can set the mirrors to fold in when locked I now look to see if the mirrors have folded in. I went to the local dealer and the advise was as follows, Don't let the wife carry the spare car in the car at the same time. make sure the person with the key leaves the car last and the drivers door is the last one to close, do not walk away from the car too quickly or the car will not know I have left, last but probably the only real answer is to turn off the car walkaway self lock and use the remote key, so why buy a car with the function???