Several months ago, Chromecasts got a new feature that would allow anyone to control any Chromecast device on the same Wi-Fi network. But what this also meant, was that anyone on your Wi-Fi network would be able to see what app you were casting from and the notification would appear during active use of the Chromecast. If you wanted to cast privately, there was no way since Google didn't include a way to remove the cast notification. That is, until today. Google has updated its Chromecast support page with new instructions on how to turn off the "Cast media control notification" that would...
Source: http://www.gsmarena.com/googles_cast_notification_can_finally_be_disabled-news-25195.php
