Google had announced this update along with updates to the GMB app, and I posted that here, on October 29, 2014.
I first spotted these in the wild around November 10, 2014 on a client’s GMB page.
This is what a review notification looks like:
Clicking on “respond now” takes users to the reviews section of their GMB dashboard where they can respond to reviews left for them on Google.
Tips like this as I run across them will be posted here or in the Google+ GMB Community.