Yes, the headphones were OEM, and I agree, I think this is an isolated issue as I too searched online for this problem. I even asked three other people if I was crazy and showed them my phone before posting here and going to the Apple store.
The Apple tech hesitantly replaced my phone yesterday. He said I should have to pay the $79 for water damage if that was indeed corrosion, but said he and the other tech couldn't tell for sure what that was, and maybe the paint was flaking. (What?!) In all fairness, I don't know what it was either, but I told him that was ridiculous considering Apple itself has used a guy running along the SF Embarcadero in its marketing, and I have only taken it running three times.
What matters is he replaced the phone, I did have Apple Care+ and he didn't charge me for it, I did however buy new headphones. It would be one thing if I were coming up on two years of Apple Care+ and trying to game the program, but it was less than two weeks in. Fingers crossed it doesn't happen again.