I was thinking about the story that Ueshiba would disappear from several yards away. Has anyone ever had their master/sensei stand in front of them and then disappear behind you?
I have, it's not a flash of smoke and then they are gone. It's a perceptual illusion using the eyes natural blind spots. You throw atemi at their face causing them to blink and then move and suddenly you are behind them, this is taught extensively in Ba-Gua (or pa-kua) although I know other styles have it also (this is mentioned because of stories I have read saying that Ueshiba trained in that style while in China and that is where he learned many concepts of aikido that aren't found in japanese martial arts but are in the internal chinese arts)
Maybe he did move behind someone and as someone else mentioned the story just grew from there until he was 50 ft. away.