That looks very similar to what happened to my 2010 MBP after I dropped it (while it was open) in a Speck hardshell. They have since replaced the display assembly and topcase (where any noticeable damage was) for dust under the glass, and because the control key was non functioning. The marks that both the OP and I have/had are the result of stress being put on the aluminum.
Pic:
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Edit: and yeah, it had to catch the light just right to be seen.
But you see unlike you, I have not dropped or mistreated the computer in any way. It has not left my desk and it's literally barely been used.