Thanks for all the thoughts. To the post above about the camera, that is correct. And I think I have figured it out. The additional collection of oil and grime happens where the cutouts for a screen protector would be. I noticed it near the mic too. I am guessing the phone used to have a protector on it and the exposed parts have minor flaws that collect more stuff now than the rest.
To that end, slight pivot on topic. Does any one have a favorite screen protector? I used a Zagg before, but I haven't seen any good reviews across the board for the Maxx.