What was Vader expecting to find when he was stepping on Obi Wan’s cloak after he had struck Kenobi down?
Some pocket sand? A hole in the floor where Obi Wan had disappeared down? A note with the message “look behind you, the Rebels, the Princess and the two familiar droids are escaping, and you’re still standing here stomping on my cloak”?
Or something else?
All joking aside, Vader was probably just perplexed that his mortal enemy vanished for no apparent reason just when he was about to kill him.
Actually, I never thought about this until now, but that might be part of the reason why Lucas retconned it so that Obi-Wan was the first Jedi to vanish after dying. If it was a normal thing for Jedi, there would be no reason why Vader was so perplexed.