system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] What did Vader, and more importantly Sidious, think Obi-Wan had actually done when he vanished?

It just seems to me that doing something like that, vanishing into nothing, would have seriously alarmed Sidious; a Force power that he knows nothing about. At the very least I'd be suspicious that somehow Kenobi had escaped and/or mastered some obscure Force power that allowed him to teleport a short distance. On the other hand some Force users were known to mysteriously vanish upon death.

So would Sidious be particularly alarmed? Would he be suspicious that somehow Obi-Wan's death was an illusion and he'd be coming back?

Your thoughts, appreciated as always.
gpt
If we're talking Old Canon, then it's likely that Sidious was aware that Obi-Wan became one with the Force, since it's not a unique ability in old continuity - many jedi over the time who die at peace with themselves and enlightened have vanished upon death. 

Vader was shown poking Obi-Wan's robes, just to be sure that he didn't suddenly turn into the Ant-Man or something, but ultimately he should be able to sense how his life was extinguished, and this is not a trick. 

In new canon it's a power unique to Yoda, Obi-Wan and Luke (and a little bit of Qui-Gon), so probably yeah, it would be somewhat  disturbing for the sith lords that their enemies achieved ability to cheat death in any way.