I've noticed this discoloration / stains / burn marks along the edges of the Quest Joint Airlock thermal cover on the hatch before, but they seem even more pronounced and contrasting with the cover's natural color now, during the last US EVA-24 of the Expedition 38 crew (Rick Mastracchio as EV1, Mike Hopkins as EV2, and Koichi Wakata as Canadarm2 operator and support inside the station). For example, this is one of photographs taken during EVA 24 as the two spacewalkers egress:
Rick Mastracchio as EV1 (red stripes) and Mike Hopkins as EV2 (no stripes) leaving the airlock during US EVA-24 (Credit: NASA)
The thermal cover is visibly discolored along its edge, gradually pronounced towards the outermost edge. I'm not sure if this is some sort of oxidation, chemical burn of thermal insulation materials as the airlock gases escape, discoloration of some materials due to exposure to the Sun as it remains open during the EVAs, or something else?
What is the cause of discoloration along the edges of the Quest Joint Airlock thermal cover?