A user who is not identified in the system. This wording is sometimes used in the error message for an undelivered e-mail message. Of course, "unidentified user" would be clearer, but why make things simple?
"ambiguous user." YourDictionary, n.d. Web. 26 May 2018. <http://www.yourdictionary.com/ambiguous-user>.
ambiguous user. (n.d.). Retrieved May 26th, 2018, from http://www.yourdictionary.com/ambiguous-user