Back cover discoloration?

[doisy]

I'm not sure if this is even a discoloration issue, but I noticed those "white marks" on the back cover on my phone. Just curious to know if any of you guys happen to have these as well, as I can't remember of putting my phone on compromising places. I'm kind of fearing that all thhat overheating may have caused them.



(they are only visible under direct light, I marked them a bit more than how they actually are so you could see what I mean and compare with yours).