Depois de Harry & amigos invadiram o cofre do Lestrange's Gringotts e pegaram a taça da Lufa-Lufa. Quando Voldemort descobriu, ele matou todos que entregaram a mensagem em Gringotes.

What did you say to me?" His voice was high and cold, but fury and fear burned inside him. The one thing that he had dreaded, but it could not be true, he could not see how...

The goblin was trembling, unable to meet the red eyes high above his.

"Say it again!" murmured Voldemort. "Say it again!"

"M-my Lord," stammered the goblin, its black eyes wide with terror, "m-my Lord... we t-tried to st-stop them... Im-impostors, my Lord... broke -broke into the Lestranges' vault..."

"Impostors? What impostors? I thought Gringotts had ways of revealing impostors? Who were they?"

"It was... it was... the P-Potter b-boy and the t-two accomplices..."

"And they took?" he said, his voice rising, a terrible fear gripping him, "Tell me! What did they take?"

"A... a s-small golden c-cup m-my Lord..."

The scream of rage, of denial left him as if it were a stranger's. He was crazed, frenzied, it could not be true, it was impossible, nobody had known. How was it possible that the boy could have discovered his secret?

Deathly Hallows Chapter 27

Ele percebeu que ao saber que a xícara havia sumido, em 1º de maio de 1998.

O Lorde das Trevas percebeu que era possível que Harry soubesse das Horcruxes quando soube que a taça de Helga Hufflepuff havia sido roubada do cofre em que estava.

“Alone amongst the dead, he stormed up and down, and they passed before him in vision: his treasures, his safeguards, his anchors to immortality – the diary was destroyed and the cup was stolen; what if, what if, the boy knew about the others? Could he know, had he already acted, had he traced more of them? Was Dumbledore at the root of this? Dumbledore, who had always suspected him, Dumbledore, dead on his orders, Dumbledore, whose wand was his now, yet who reached out from the ignominy of death through the boy, the boy –”
_{^{- Harry Potter and the Deathly Hallows, Chapter 27 (The Final Hiding Place)}}

Isso aconteceu um dia antes da batalha final, que J.K. Rowling disse que aconteceu no dia 2 de maio.

Victoire, who’s in the epilogue, was so named because she was born on the anniversary of the battle that finished it all. Which is the second of May, if anyone’s been paying attention.
_{^{- J.K. Rowling: A Year in the Life}}

O ano seria 1998, como é mencionado em "Harry Potter e a Câmara Secreta", dizendo que Nick quase sem cabeça havia sido um fantasma por quinhentos anos, já que é o quinhentos "dia da morte" dele.

“Well, this Hallowe’en will be my five hundredth deathday,’ said Nearly Headless Nick, drawing himself up and looking dignified.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 8 (The Deathday Party)}}

Também é mencionado que ele morreu em 1492, o que significaria que o ano era então em 1992.

“Sir Nicholas de Mimsy-Porpington
died 31st October, 1492”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 8 (The Deathday Party)}}

Isso faria o segundo ano de Hogwarts de Harry entre 1992 e 1993, então os anos em que os livros finais seriam lançados de 1997 a 1998, então a batalha aconteceu em 1998. Portanto, o Lorde das Trevas começou suspeito que Harry sabia sobre as Horcruxes em 1 de maio de 1998.

Quando Harry, Hermione e Ron viajaram para Gringotts para roubar a Horcrux do cofre dos Lestranges. O grupo não tinha motivos para estar no cofre dos Lestranges, a menos que eles soubessem sobre a Horcrux, então Voldemort facilmente descobriu suas intenções.