A supposed person that exudes a presence but isn't definitively present, or even existent; much like 'seeing' a missing button: an object that you may assume exists and is somewhere, but there isn't any concrete evidence that this is so.
A missing button is NOT simply someone you haven't met yet. A missing button is renowned on some level, but validated by very few (if anyone).
"Torin has mentioned Erica from time to time for the past couple years and no one has ever met her. Anyone find that odd?"
"Well, we did group chat with her on AIM that one time for 10 minutes. I feel like he might have shown me a picture of the side of her head once, I can't remember."
Erica is a missing button.