User is logged in - CI destroys session - how to know if user was logged in?

[eluser]CloppiWeb[/eluser]
Hey everybody.

I'm currently doing my bachelor thesis and my subject is framework for a site that runs auctions. I still have many months ahead before the product is finalized and ready but very close of launching beta version for public use as a part of testing.

I'm always considered about user experiment and this case involves notifying user what is happening in the application. So, my problem, which I have been solving for the last three days is:

Users current state is saved in the session, from where the application checks if user is logged in or not. Ci takes care about the session validation. I want to know when Ci destroys the session, whether or not user was logged in. I'v tried extending the input and session class, I've also tried hooking pre_system but I just cant catch the change. This is a minor factor, I just thing it would bee cool if user was notified after the session is destroyed that system logged you out.