Very strange issue here. Around the time the 5s came out I went into AT&T to grab a new phone. For some reason I couldn't get logged into my apple account during setup so the AT&T employee completed the activation while using their account.
Ever since then when I did factory resets and after I entered my own apple credentials it would always ask me to log in with their account as well. Didn't bother me too much and I figured it was something particular to the phone.
However, I just bought a new 7 and the same issue has come up! After I log in with my account it asks me to log in with this strangers account. Obviously the accounts are linked in some fashion but I've checked every single account setting I can and I have no idea how this could have happened.
Anyone have any ideas?
Edit: It's not a used phone, but were new out of the box at AT&T. Also, doesn't sounds like it's an activation lock because it has followed me across phones. Definitely seems like an iTunes/Apple ID account issue.
Here's the popup that shows up right after a restore and when I open various apps following a restore. For what it's worth, this appears to be their personal email and not a corporate email: