TL; DR:
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Ele não estava tão presente , como ele reapareceu momentaneamente para agarrar a Bellatrix, como podemos inferir das declarações do funcionário da MoM .
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No entanto, NÃO vemos esse reaparecimento na narrativa (inclusive do ponto de vista de Harry) .
Detalhes:
Por um lado, ele claramente desapareceu do corredor antes de possuir Harry ... E não reapareceu mais tarde ... E os livros explicitamente dizem que ninguém estava no corredor naquele momento :
For a few seconds Voldemort was visible only as a dark, rippling, faceless figure, shimmering and indistinct upon the plinth, clearly struggling to throw off the suffocating mass. {{ This is the last moment he's visible - DVK }}
Then he was gone and the water fell with a crash back into its pool, slopping wildly over the sides, drenching the polished floor.
'MASTER!' screamed Bellatrix.
Sure it was over, sure Voldemort had decided to flee, Harry made to run out from behind his statue guard, but Dumbledore bellowed: 'Stay where you are, Harry!'
For the first time, Dumbledore sounded frightened. Harry could not see why: the hall was quite empty but for themselves, the sobbing Bellatrix still trapped under the witch statue, and the baby phoenix Fawkes croaking feebly on the floor
ENTÃO, Harry fica possuído:
Then Harry's scar burst open and he knew he was dead: it was pain beyond imagining, pain past endurance.
... skip the paragraph's about Harry's attempted possession
And as Harry's heart filled with emotion, the creature's coils loosened, the pain was gone; Harry was lying face down on the floor, his glasses gone, shivering as though he lay upon ice, not wood . . .
E, ENTÃO, as pessoas do Ministério, incluindo Fudge , dizem que viram Voldemort . Mas quando??? De citações anteriores, ele desapareceu antes de aparecerem! Contradição!
And there were voices echoing through the hall, more voices than there should have been . . . Harry opened his eyes, saw his glasses lying by the heel of the headless statue that had been guarding him, but which now lay flat on its back, cracked and immobile. He put them on and raised his head a little to find Dumbledore's crooked nose inches from his own.
'Are you all right, Harry?'
'Yes,' said Harry, shaking so violently he could not hold his head up properly. 'Yeah, I'm ... where's Voldemort, where who are all these... what's '
The Atrium was full of people; the floor was reflecting the emerald green flames that had burst into life in all the fireplaces along one wall; and streams of witches and wizards were emerging from them. As Dumbledore pulled him back to his feet, Harry saw the tiny gold statues of the house-elf and the goblin, leading a stunned-looking Cornelius Fudge forward.
'He was there!' shouted a scarlet-robed man with a ponytail, who was pointing at a pile of golden rubble on the other side of the hall, where Bellatrix had lain trapped only moments before. 'I saw him, Mr Fudge, I swear it was You-Know-Who, he grabbed a woman and Disapparated!'
'I know, Williamson, I know, I saw him too!' gibbered Fudge, who was wearing pyjamas under his pinstriped cloak and was gasping as though he had just run miles. 'Merlin's beard here here! in the Ministry of Magic! great heavens above it doesn't seem possible my word how can this be ?' (HP: OoTP, Ch 36: The Only One He Ever Feared)
A única explicação parece ser que Voldemort reapareceu - não mostrado na narrativa do livro, texto Harry-POV - após a posse, para agarrar Bellatrix, assim como os funcionários do Ministério chegaram.