User blog comment:FairyRaven/Fairy Tail Chapter 360:The White Inheritance, Review and Commentary/@comment-9468724-20131117114459/@comment-7418318-20131117123751

I've already put up the same theory. While I think it's unlikely, it is possible.

Maybe Mavis called it Lumen Histoire to make sure little leaks of information where only the name slipped out, but not the nature of it (like Ivan, who learned the name, but not what it was) wouldn't cause any problems. Or either the council or Mavis didn't know about the "true name" of it (which one depends on which whether the true name is Lumen Histoire or Faith/White Inheritance).

It certainly would make for an interesting situation if Yajima turned out to know about Faith and then finds out that it's been hidden under the very guild of which he was a member without him ever knowing.