I own a late 2013 MBPr and recently noticed shadow stain marks at the bottom-left of the screen. I made an appointment with the Apple agent/supplier. After a brief visual inspection and MRI test, a new retina integrated unit was ordered and fitted next day free of any cost to me. The problem is a defective anti-reflective display coating whereby marks and stains appeared on the screen. The Apple agent confirmed that Apple acknowledged the issue and had launched a Quality Program to replace defective screens... I hope this helps.