Hi again! I'm almost done!
Here is the thing...I'm able to successfully listen to the event from the STOP CASTING  button using:
this._session.addUpdateListener(function listenerSession() {
   if (this._session.status === window.chrome.cast.SessionStatus.STOPPED) {  /* here *\ }

But I'm not able when I touch the X in the cast notification. As far as I understand that stop the session too.

Thanks !!