The Sony Hack: Why We Know Its an Inside Job

Multiple insiders have pointed out that IT security was basically not existing at Sony, so I doubt that they had a login notifier.

Furthermore, there are a number of ways of logging in which would not trigger such a notifier - e.g. by uploading a "webshell" and going to root from there using su with the password. Or using passwordless sudo or bad SETUID programs (I have seen a couple of systems with passwordless sudo, set up by REPUTABLE companies!).