How did people know so fast that Voldemort died/vanished/disappeared?

Question

This question has been treated (for example here or here) but I want to know a very specific detail that struck me as I re(-re-re-re)-read the first book.

People celebrated the defeat of Voldemort hours after he tried to kill Harry. But how did they know he died/vanished/disappeared? I mean, why didn't they think "Oh well, he just wanted to kill the Potters for they were members of the Order, and now they're dead. He left the baby because the baby was not a threat. He'll probably kill someone else tomorrow."

At this moment, very few people know about the prophecy. But wizards have been shown celebrating in the whole country. How did they know so fast he was not around anymore? Why wouldn't people think he just left the place after killing the Potters?

Answer 1

Voldemort was not the kind to leave

the baby because the baby was not a threat.

That aside per

http://harrypotter.wikia.com/wiki/Imperius_Curse

and as suggested by NominSim.

as happened after the final defeat of Voldemort and the Death Eaters in the Battle of Hogwarts, when all of the people across the country who had been Imperiused by him or his supporters were released from the spell

Answer 2

Some of the Dark Lord’s spells, like the Imperius Curse, broke.

Hagrid says the proof he personally believes in that the Dark Lord has lost his power and wasn’t simply waiting is that people came out of their ‘trances’ after the Dark Lord disappeared.

“That’s the biggest myst’ry, see … he was gettin’ more an’ more powerful – why’d he go?

‘Some say he died. Codswallop, in my opinion. Dunno if he had enough human left in him to die. Some say he’s still out there, bidin’ his time, like, but I don’ believe it. People who was on his side came back ter ours. Some of ’em came outta kinda trances. Don’ reckon they could’ve done if he was comin’ back.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 4 (The Keeper of the Keys)}}

People who witnessed others coming out of their trances might figure out that the Dark Lord was indeed weakened, and some would probably spread the news, and it could be seen by the wizarding world as proof. The only other time that something similar happened was when the Dark Lord was permanently killed, so it seems to take a drastic weakening to cause this to happen.

“He must speak to the bereaved, clasp their hands, witness their tears, receive their thanks, hear the news now creeping in from every quarter, as the morning drew on, that the Imperiused up and down the country had come back to themselves, that Death Eaters were fleeing or else being captured, that the innocent of Azkaban were being released at that very moment, and that Kingsley Shacklebolt had been named temporary Minister for Magic …”
_{^{- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)}}

People who witnessed this occurring would have reason to believe that the Dark Lord had been weakened, since it wouldn’t be happening if the Dark Lord was still at the height of his power.

Answer 3

Two things to note while answering your question.

1. Harry's family died in the PM on Halloween, Harry does not arrive at the Dursleys until the PM of the following day, November 1st.

2. We only have clear celebration starting the morning of November 1st, Mr Dursley notices it on his way to work.

Taking these things into account, the Daily Prophet was most likely to culprit of the wide spread knowledge of at least Voldemort's disappearance, if not outright stating he was gone.

But to further speed up the spread of information, the Prophet is also known to release an evening issue.

as Snape unrolled today’s issue of the Evening Prophet. “You were seen,” he hissed, showing them the headline: FLY- ING FORD ANGLIA MYSTIFIES MUGGLES. He began to read aloud:

We know Voldemort's attack came while Harry was still awake for the night, and he lived in a Wizarding community so other wizards would have been aware of the house being blown up. Not hard to imagine after that a quick owl to the Prophet, and an evening/late night special report to get sent out asap.

Note again, the celebration is still semi speculative, as Professor McGonagall states.

‘What they’re saying,’ she pressed on, ‘is that last night Voldemort turned up in Godric’s Hollow. He went to find the Potters. The rumour is that Lily and James Potter are – are – that they’re – dead.’

Beyond that, the scar on Harrys forehead, and the house blowing up, and lack of a dark mark in the sky all shed light on something amiss from the usual order of business. Even by the time Harry is 11 Hagrid says speculation still abounds.

‘But what happened to Vol– sorry – I mean, You-Know-Who?’

‘Good question, Harry. Disappeared. Vanished. Same night he tried ter kill you. Makes yeh even more famous. That’s the biggest myst’ry, see ... he was gettin’ more an’ more powerful – why’d he go? ‘Some say he died. Codswallop, in my opinion. Dunno if he had enough human left in him to die. Some say he’s still out there, bidin’ his time, like, but I don’ believe it. People who was on his side came back ter ours. Some of ’em came outta kinda trances. Don’ reckon they could’ve done if he was comin’ back.

Answer 4

Remember that voldemort only came to Godrics Hollow to finish what the prophecy had stated that the boy born on 31st july would be the end of his legacy.This made his intentions quite clear to his fellow death eaters.Also consider the possibility that the Death eaters who were captured by aurors may have leaked the imformation to avoid azkaban like 'igor karkaroff'

Answer 5

The Dark Marks on the Death Eaters' wrists disappeared, only to re-appear nearing the end of the tri-wizard tournament. It is posible that Snape informed Dumbledoore on the dissappearance of his Dark Mark, and the ramifications of this fact.