I have a icon that looks like a slanted exclamation point on my task bar after the 4.3 update. When i am on Blinkfeed or using the phone it goes away but reappears on any other home screen. No big deal but curious what it is for.
my guess is prior to 4.3 your notification bar wasn't transparent, and when you are in other apps your wall paper isn't showing so the light reflecting off water in the depressions between the bricks isn't there to make the shape you are seeing as a slanted exclamation mark.
change your wallpaper... it won't be there any longer. I'd bet my house on it.