Nowhere on that banner does Facebook make it clear that there was recently a severe security issue that may have resulted in the loss of personal user information (Making it much less likely for the user to actually click 'Learn More'). It's misleading to title this with just "An Important Security Update" and make it seem like they've just updated their systems. No mention of the recent compromise until you click 'Learn More'.
https://www.facebook.com/help/2687943754764396?ref=comms
Which is the issue at hand.
Also, it's "100%" the banner I saw for several days last week. Make of that what you will. I didn't click it so I don't know what it pointed towards.
I just checked fb, and went quickly to my first notification. I didn't really register what the banner was until maybe a second after it loaded - at which point I had already clicked on my first notification. By that point, the banner is gone forever. I can't find any way to get it back.
It's so, so easy to miss this message.