How did Voldemort know about Harry’s protection with the Dursleys?

Question

During his – ahem – absence, Voldemort manages to figure out the reason why the curse he used to try to kill young Harry rebounded on himself: Lily had been given a choice to save herself or die protecting Harry, and her sacrifice had given Harry an invincible protection. By the time Voldemort returns, this is probably known by enough people that it’s not wholly unexpected that Voldemort finds out about it too.

Less known – much less known – is the fact that, within 24 hours of Voldemort’s failed attack, Dumbledore, probably the first to realise why the spell had failed, had used the lingering protection afforded by Lily’s sacrifice to protect Harry against Voldemort at the Dursleys’ house while he can still call it his home. As far as I can recall, Dumbledore never confided in anyone apart from Petunia Dursley that this protection had been put in place.

And yet, when Voldemort returns, he obviously knows about it:

‘But how to get at Harry Potter? For he has been better protected than I think even he knows, protected in ways devised by Dumbledore long ago, when it fell to him to arrange the boy’s future. Dumbledore invoked an ancient magic, to ensure the boy’s protection as long as he is in his relations’ care. Not even I can touch him there …’

^{Harry Potter and the Goblet of Fire, ch. “The Death Eaters” (my emphasis)}

How did Voldemort find out about Dumbledore’s ‘old magic’ protection of Harry while he’s in the Dursleys’ care?

Answer 1

The Dark Lord likely figured it out himself, one way or another.

The Dark Lord was able to figure out how Lily’s sacrifice residually protected Harry so that he couldn’t touch him, and he also knew of the protection that the sacrifice cast originally to shield Harry from the Killing Curse. (He seems to have figured all of this out for himself, from the evidence he had, rather than being told by any specific person.)

“Voldemort raised one of his long white fingers, and put it very close to Harry’s cheek. ‘His mother left upon him the traces of her sacrifice … this is old magic, I should have remembered it, I was foolish to overlook it … but no matter. I can touch him now.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

Part of the charm was placed upon Harry himself, so there may be traces on him of that as well.

“She may have taken you grudgingly, furiously, unwillingly, bitterly, yet still she took you, and in doing so, she sealed the charm I placed upon you. Your mother’s sacrifice made the bond of blood the strongest shield I could give you.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 37 (The Lost Prophecy)}}

The Dark Lord certainly knew about it by the time he and Wormtail were plotting to have Barty Crouch Jr. abduct Harry away from Dumbledore at the Triwizard Tournament, since he based his plan knowing Harry would be safe with his relations. It may be as simple as him finding out from Wormtail (who spent years as the Weasleys’ rat, and more than two years as Ron’s rat so in close proximity to Harry) that Harry lived with his aunt and realizing Dumbledore would have used the sacrifice to protect him while he was there.

“I have my reasons for using the boy, as I have already explained to you, and I will use no other. I have waited thirteen years. A few more months will make no difference. As for the protection surrounding the boy, I believe my plan will be effective.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 1 (The Riddle House)}}

It’s possible he figured it out by the evidence he knew of. It’s also possible he detected traces of this magic on Harry somehow, the same way he figured out about about the sacrifice making his spell rebound then making him unable to touch Harry. Another way he could have figured it out is if he’d ever tried to get at Harry there. We know, at least by the time the charm is close to breaking, that the Dark Lord and the Death Eaters knew around where the Dursley house was.

“They might not be able to get at you or this house while your mother’s charm holds, but it’s about to break and they know the rough position of the place.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 4 (The Seven Potters)}}

Dumbledore didn’t seem to expect the Dark Lord wouldn’t know.

Though Dumbledore chose it to protect Harry, he didn’t seem to think that the Dark Lord wouldn’t know what he’d done - he referred to it as ‘magic of which he knows, which he despises’, so he doesn’t seem to expect that the Dark Lord won’t know he’s protected with his aunt.

“I knew that Voldemort’s knowledge of magic is perhaps more extensive than any wizard alive. I knew that even my most complex and powerful protective spells and charms were unlikely to be invincible if he ever returned to full power.

‘But I knew, too, where Voldemort was weak. And so I made my decision. You would be protected by an ancient magic of which he knows, which he despises, and which he has always, therefore, underestimated – to his cost. I am speaking, of course, of the fact that your mother died to save you. She gave you a lingering protection he never expected, a protection that flows in your veins to this day. I put my trust, therefore, in your mother’s blood. I delivered you to her sister, her only remaining relative.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 37 (The Lost Prophecy)}}

It doesn’t seem like Dumbledore expected the protection he gave Harry to stay secret from the Dark Lord - he knows the Dark Lord knows of that type of magic.

Answer 2

It is possible that Dumbledore told the Order why he left Harry with the Dursleys and that Wormtail found out while living with the Weasleys, but on the whole that seems unlikely.

My conclusion is that Voldemort and Wormtail probably visited Little Whinging en route to the Riddle House. Even in his weakened state, Voldemort would have been able to sense the nature of Dumbledore's magic, in much the same way that Dumbledore was able to sense the nature of the magic used in the Horcrux cave.

Answer 3

Either he checked the Dursleys' home for protection (whether he went himself or sent Peter or Barty) or he deduced it.

As for deducing, at the end of PS in front of the mirror, Quirrell tries to attack Harry.

Quirrell raised his hand to perform a deadly curse, but Harry, by instinct, reached up and grabbed Quirrell’s face –

Quirrell rolled off him, his face blistering too, and then Harry knew: Quirrell couldn’t touch his bare skin, not without suffering terrible pain – his only chance was to keep hold of Quirrell, keep him in enough pain to stop him doing a curse. (PS)

Quirrell died from the burns, but Voldemort escaped. He had two years to think about the reasons why this happened. Later he says

"His mother left upon him the traces of her sacrifice … this is old magic, I should have remembered it, I was foolish to overlook it … but no matter. I can touch him now." (GoF)

This "I should have remembered" means that he knew about this kind of protection even before he killed the Potters, but for some reason didn't think about it at that time.

Dumbledore says

“I knew that Voldemort’s knowledge of magic is perhaps more extensive than any wizard alive. (OotP)

So if Voldemort knew about this protection that saved Harry, he probably also knew that it could be used to protect Harry at his home. And Voldemort knew Dumbledore for about fifty years, so he knew that it would be Dumbledore's style to extend this love protection.