[quote=Psyduck;35560]I think it's Lupin.
Imperius curse, traitor, I don't know what.
Something is wrong.
Something's been wrong all along.
Page 204, where Lupin "proves" that it's really him --
That's all common knowledge. Snape, at least, knows all of that.
Harry didn't really ask him anything.[quote]

Where exactly did Snape get his information? I mean, I would assume that he got it from Dumbledore's portrait, but how would the portrait know about it? Especially something like the real date of Harry's departure. If Dumbledore had been visiting other portraits of his, he would only know the false date, right? Lupin being an informant for Snape fits.