Blue Triangle, top left, next to BFB.
I have no idea what it means, or why it is there. I thought it was a bug.
According to it means that an app needs attention, but I don't know which one.
I clicked on all the open apps in the launcher in turn until the blue triangle went away.
So this isn't really a technical bug, its more of a usability problem. If there is some sort of indicator, it needs to be clear what it is indicating. Or maybe have someway of discovering what it is for - a label that pops up when you mouse over it? Open the app that requires attention when you click on the blue corner?
Maybe after a while users will learn to look at the launcher when they see the blue triangle by the BFB, but initially its baffling.