Yesterday I noticed my late 2013 iMac has image retention too...
It was the first time I used the mac the entire day. And I think it happens when the screen warms up.
I browsed the web for an hour and then I opened up photoshop and saw my browser window's ghost image on the gray background of the empty workspace. It stayed on there for a very long time and only went away after activating the flurry screensaver for a few minutes.
I don't know what I will do with this iMac. I love to work on it and I never saw the image retention before. I use it for 3 weeks now.
I checked if I saw image retention a minute ago but have no ghost images right now.
If it only happens after 12+ hours of use I'm not going to try to exchange it. Chances are I will get a screen in return with a yellow tint or light bleeding.
I also only notice this on plain grey backgrounds.
I'm a little bit disappointed in Apple. This is not the first screen issue I'm experiencing. I have a Macbook Pro 17" with severe banding. I can live with it because the laptop is property of the company I work for and I did not pay for it.
I also bought an iPad 3 which had severe light bleeding.
(
https://forums.macrumors.com/attachment.php?attachmentid=293642&d=1310594235). Luckily I managed to have it exchanged for a better one.
I'm glad I spend a little extra on Apple Care for this machine. Sooner or later I will need it. I'm almost sure problems will occur within the next three years.
Apple ≠ quality these days anymore...