I have the 15" MacBook Pro, Retina, Mid 2012 also, same exact model. Kind of the same thing is happening with me. I will intermitentyl get a flickering on the bottom 1/3 of the screen, all the way across. It flickers/flashes on and off on the bottom half or third of the screen, when I am typing or when I am perfectly stil, doen't really matterl. Only happens for a split second, but can do it multiple times, or sometimes not at all. I can go tens of minutes without i hapenning, but I can also go tens of seconds with it happening multiple times.
It never goes black or grey or "out" at all, just randomly flickers throughout the day.
I do get burned-in images all the time, but they disappear within seconds. But this flickering is definitely worrysome. I did get the the Apple Care, so I am not too worried as Apple is usually good about this kind of thing, but I do have a similar problem to you and wanted to document this. I didnt find many other people oiut there with this problem, so let me know what has gone on since your post on the New Year. Any solution? Did you call Apple?
2012 Retina MacBook Pro
500GB Flash Drive, 16GB memory
2.6 Ghz Intel Core i7
You need to call AppleCare and get the screen replaced. Both these issues are due to the screen used on your model coming from the manufacturer that's had production problems with these screens. Apple knows about the issue, but they're not going to call you about it. YOU have to be the one to initiate the call and warranty fix. Good luck.