If they're going to steal the keys, no car is secure.
Thought we were talking joyriders here? Stealing a new car to break it up for spares is rather high end criminals, surely?
You can only take reasonable precautions. You could stop going to pubs where you know there are pickpockets, for example. ;-)
Probably a 7 Series. That's why they go wrong so much - they try out the new bits and when they're fixed they filter down to the others.