Ive just had this with mine, found it to be a lazy drivers door c/locking motor which wasnt going fully down, perhaps you should use the deadlock button for now until youve found the culprit.

Thanks guys.  The missus tends to hit the deadlock button routinely, and keeps the keys in her bag rather than her back pocket.  So I'm a bit mystified.

My best explanation is that the locking process was omitted; but she isn't too 'scatty', so when she says it was locked, I tend to believe her.

Struggling for a rational explanation though.