I went through that activation lock when I traded in my 6S Plus under the AUP for 7 Plus. Took Apple Store over an hour to clear the lock.Happened to the 6S Plus I traded back in as part of the iPhone Upgrade Program. I was the only owner but it showed it was locked to k*********@icloud.com which is not my address.
Apple removed the lock in store, took the phone back and all was well.