Who is the "Unknown External User"

Whether it's an Unknown External User listed as a collaborator or an unknown user who viewed a file, there's always a reason for it. 

Typically we will encounter "Unknown external user" when a user the file had been shared with has been deleted from the system. Because the user's domain is now registered as 'null' and 'null' is not an internal domain, it is considered and treated as an external user. 

Alternatively, if a file is shared with an external user (box,dropbox, office, etc) - collaborator details aren't available until the user actually accepts the invite and accesses the file.