Received my iMac 27" around a month ago and today I noticed the IR.
I also performed a test and BAM you can clearly see it

.
I don't want to start driving around with this iMac to the Apple stores or be without one for a while.
Will try to log a support call with Apple to get it replaced by sending a new one but this is kinda sad to see. Was so happy until now and once you notice this problem you just keep looking at it.
My wife doesn't really understand what the big deal is even when I show it to her, can't believe she finds this normal that you can see your finder window which is closed long time ago still in the background.