Yikes.... My 2008 (57) Captiva just has the same (well sort of the same) problem.
Only use one key, spare is in my safe, never ever used. My key has started playing up, cant open or lock sometimes. Thought it was the battery. So got the spare key out of the safe, and that is doing the same thing.
Not noticed it affecting other cars though...
be interested to find out what you discover....