Why was Vader surprised that Obi-Wan's body disappeared?

Question

OK, I understand that Obi-Wan turned into a force ghost when Vader struck him down, but why was Vader surprised by the lack of a body? It's not the transition into a force ghost but Vader's reaction that bugs me.

Answer 1

At the end of Revenge of the Sith (2005), Yoda talks to Obi-Wan Kenobi about meeting an old friend, his old master, Qui-Gon Jinn, who we know became a force ghost.

02:04:50 - Yoda: One who has returned from the netherworld of the Force.

02:04:53 - Yoda: Your old master.

02:04:56 - Obi-Wan: Qui-Gon?

02:05:00 - Yoda: How to commune with him I will teach you.

This is the first time, chronologically in the films, that the audience and any of the relevant main characters are introduced to the concept of force ghosts. Indeed Obi-Wan is surprised to hear about Qui-Gon in this scene.

Presumably, Anakin had also never heard of this concept of force ghosts before, or even if he did, at the very least could not fathom that Obi Wan could achieve such a feat. We can see from his reaction that he is unable to comprehend what just happened, he probably assumes that he defeated him, confused about the phenomena of the body disappearing, and later concludes that he died.

The following dialogue is from their lightsaber duel scene in Star Wars (1977).

01:27:08 - Ben: You can't win, Darth. If you strike me down...

01:27:12 - Ben: I shall become more powerful than you can possibly imagine.

01:27:22 - Vader: You should not have come back.

From Anakin's line (and his physical reaction, poking around the robes with his feet) there is evidence to suggest that he was, at least at the time, oblivious to the possibility of Obi-Wan becoming one with the force (turning into a force ghost).

Answer 2

He didn’t understand what had happened

The knowledge of how to become a Force ghost was known to very few. Anakin certainly did not know it back during the Clone Wars era:

ANAKIN: Everything that we know about the Force tells us that an individual retaining their identity after death is impossible.

YODA: Everything that we know, yes. But what about what we know not? Hmm?

_{The Clone Wars: “Voices”}

Qui-Gon learned it from the Force Priestesses, who had preserved this ancient art. Yoda learned it from Qui-Gon, with some help from the priestesses.

YODA: Now does my training begin?

PRIESTESS: The one you know as Qui-Gon Jinn will commune with you and guide your training. Like us, you shall learn to maintain your consciousness after death.

_{The Clone Wars : “Sacrifice”}

If even Yoda, Grand Master of the Jedi Order, was unaware of the possibility of becoming a Force ghost, surely Anakin, later Vader, also was. One might imagine that Palpatine might have told him, but Palpatine almost certainly was also unaware. Palpatine sought to cheat death, in the tradition of Darth Plagueis, but not by becoming a Force ghost: he wanted to make himself physically immortal and rule the temporal world. He wanted to control the midi-chlorians of life, not survive as a disembodied spirit and ultimately merge with the Cosmic Force. As Yoda says, after acquiring the knowledge of the Force Priestesses:

YODA: Yet, open to us, a path remains that unknown to the Sith is. Through this path, victory we may yet find. Not victory in the Clone Wars but victory for all time.

_{The Clone Wars: “Sacrifice”}

Vader’s behavior and words bear this out. He appears to be entirely unaware that Obi-Wan exists as a Force ghost, even as of The Empire Strikes Back. He believes that Obi-Wan can no longer help Luke, for example:

“He could destroy us,” the Emperor croaks.

For a moment, Vader does not speak. When he does, his rich voice is slow and deliberate. “He is just a boy. Obi-Wan can no longer help him.”

_{The Empire Strikes Back: So You Want to Be a Jedi?}

He also claims that Obi-Wan was "destroyed" and that there is no life after death:

“Don’t let yourself be destroyed, as Obi-Wan did.”

Vader’s black helm shimmers in the dim light of the reactor core. His cape sways softly, like silk, with each step he takes. “Do not believe the Jedi lies. There is no life after death. There is only death.” Behind his mask, he is smiling. He must be.

_{The Empire Strikes Back: So You Want to Be a Jedi?}

In particular, as Sith, both Vader and Palpatine believed that the Jedi belief in life after death (even as part of the Force) was mistaken. They didn’t even believe that one could become one with the Force, let alone retain one’s identity.

SITH: We are the Sith.

YODA: Afraid of you I am not.

SITH: We shall see. There is no life after death. Only nothingness awaits you, Jedi. Your fear feeds our hunger for power. They will know you are here. We will tell them. You will die and be nothing.

_{The Clone Wars: “Sacrifice”}

Now, what did he think had happened? It’s not clear. Probably he could sense through the Force that Obi-Wan was no longer there, so he did not believe that he had, for example, turned invisible. He was surely too arrogant to believe that Obi-Wan had, as he suggested, become “more powerful than [he] could possibly imagine” (nor would he have understood what that meant, even if he credited it). Perhaps he believed that Obi-Wan had used some trick to destroy himself, committing suicide rather than let Darth Vader kill him.

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One might recall that Qui-Gon Jinn appeared to Anakin in the Mortis arc of The Clone Wars, and wonder why this hadn’t convinced him of the possibility of survival as a Force spirit. In that same arc, though, the Son, a powerful entity of the Dark Side, had appeared to him in the form of his deceased mother, Shmi Skywalker! He (and even Ahsoka and Obi-Wan) would have had ample reason to suspect that the vision of Qui-Gon was simply the Son, Daughter, or Father appearing in a different form for their own goals. Even without this sort of thing going on, many Jedi suspected that Qui-Gon was a trick of the Sith when Yoda heard him.