maybe Inception will make this easier to achieve in the future... well, then maybe use some pixels on the side of the feeds/homescreen... just few of them, discretely... or app icon to get some badge or change colour... but that would mean that we would have to go to the app folder and then to scroll to the very icon - a bit unpractical... my vote goes for colon of the time format (":") to slightly change the colour after something has been filtered, and discretely change few pixels on the left side of the feeds/homescreen...