The Leviathan's Resting Place

A 3-hour flight from Seattle to Los Angeles. 

A man is seen entering a cramped bathroom of First Class -- after which he disappears into thin air. The door was under supervision the entire time.

A search of the bathroom itself turns up nothing. A search of First Class turns up nothing. A search of the entire plane turns up nothing. 

An impossibility, by all accounts. And yet, there's no doubt a human being had to have been behind it. How was it done, though? And why?