Consulting the wiki, it does state that "Snape could fly without the use of a broom or any other visible materialistic support. The only other wizard that was known to be able to do this is Lord Voldemort, who most likely taught Snape the method to do so."
Interesting. That seems to fit well enough with my theory of 'advanced generalisation of apparition-type magic' so I'll run with that hypothesis until something better or MoR!authoritative comes along.
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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