My son's iPhone 5s was locked and erased by someone he shared a cloud with during a work project. This phone was never on this man's account. I have a receipt as proof of ownership. I also have the email originally placing the Apple ID and cloud info. I bought the phone new from AT&T and had have it on my account for the past year.
The person we assume did this denies locking the device maliciously or accidentally.
Apple refuses to unlock the phone and won't even tell who or what Apple ID locked the phone.