As long as you have sent an e-mail to the phantom contact before, the iPhone stores it in its memory. For example, I have contacted sellers on craigslist via iPhone before, even though they are not in my contact list, if I go to compose an e-mail their email address will pop-up. Surprisingly enough, one of the features I missed on the iPhone when i switched to BlackBerry for a while. Chances are, you have sent an e-mail to this person in the past and forgot about it.