Hi Hector,
Firstly, logging out does not delete a session. It's just a logout. The Session only deletes when the "session time" expires. You can of course call delete session manually when the user logs out - alto ugh you can get into tricky situations doing that.
If the user turns off their browser, or navigates away from your app, or turns off their machine, or whatever, then you do not get any notification of that. In these cases the sessions are tidied up by the server after the session times out.
cheers
Bruce