Harry's parents for example. They were killed in the muggle world, meaning that sooner or later the cops would have found their bodies, no? Were the murders just ruled unsolved?
A better example though would be the deaths of wizards/witches while they're at Hogwarts. Surely they must be documented and registered as citizens in the muggle world?
Allow me to add my own data to this. This is from the conversation the Muggle Prime Minister has with "the Other Minister".
'I won't deny that morale is pretty low at the Ministry,' said Fudge. 'What with all that, and then losing Amelia Bones.'
'Losing who?'
'Amelia Bones. Head of the Department of Magical Law Enforcement. We think He Who Must Not Be Named may have murdered her in person, because she was a very gifted witch and - and all the evidence was that she put up a real fight.'
Fudge cleared his throat and, with an effort, it seemed, stopped spinning his bowler hat.
'But that murder was in the newspapers,' said the Prime Minister, momentarily diverted from his anger. 'Our newspapers. Amelia Bones ... it just said she was a middle-aged woman who lived alone. It was a - a nasty killing, wasn't it? It's had rather a lot of publicity. The police are baffled, you see.'
Fudge sighed. 'Well, of course they are. Killed in a room that was locked from the inside, wasn't she? We, on the other hand, know exactly who did it, not that that gets us any further towards catching him. And then there was Emmeline Vance, maybe you didn't hear about that one -'
'Oh yes I did!' said the Prime Minister. 'It happened just round the corner from here, as a matter of fact. The papers had a field day with it: Breakdown of law and order in the Prime Minister's back yard -'
Harry Potter and the Half-Blood Prince - p.19 - Bloomsbury - Chapter 1, The Other Minister
I think this speaks for itself really. These were wizards murdered by wizards and, clearly, you are quite right. The bodies are found and they do baffle police.
There are surely exceptions to this, though. When Mad-Eye Moody was killed, the Death Eaters "tidied up after themselves" and, obviously, in the cases of wizards who live in Muggle towns and villages and interact somewhat with their neighbours, if they die of old age, presumably it's put down to dying of old age.
As for little kids at Hogwarts, it seems kind of unlikely they would be missed.
An example of this is evident in Harry Potter and the Goblet of Fire. We're shown how the residents of Little Hangleton (Muggles) responded to the deaths and bodies of Tom Riddle Sr and his family.
Muggle police did find the bodies, and the bodies were buried by the residents of Little Hangleton. Wizarding authorities were also aware of the death, and its magical nature, and therefore investigated the Gaunts,never colluding or cooperating with the Muggle authorities - Muggles were never made aware that Morfin Gaunt was charged with the murders.
Law enforcement was confused as to the cause of death because the Killing Curse leaves no sign of damage or physical evidence. Note: This also meant there was also no evidence or signs of natural death, such as heart attack, or any organ damage or deterioration. The reason for their deaths remained a mystery.
Suspicion was cast on Frank Bryce, the gardener, but charges had to be dropped due to lack of evidence. As such, the identity of the killers was never resolved, as far as the Muggle world was concerned. Frank Bryce continued to be suspected by residents until his death many years later, for lack of a better suspect.
The police had never read an odder report. A team of doctors had examined the bodies, and had concluded that none of the Riddles had been poisoned, stabbed, shot, strangled, suffocated or (as far as they could tell) harmed at all. In fact, the report continued, in a tone of unmistakable bewilderment, the Riddles all appeared to be in perfect health – apart from the fact that they were all dead. The doctors did note (as though determined to find something wrong with the bodies) that each of the Riddles had a look of terror upon his or her face – but as the frustrated police said, whoever heard of three people being frightened to death?
As there was no proof that the Riddles had been murdered at all, the police were forced to let Frank go.
Harry Potter and the Goblet of Fire - p.9 - Bloomsbury - Chapter 1, The Riddle House
Wizarding deaths are undoubtedly inspected by the muggle police, but in the absence of a mundane cause of death, it would appear that they're often ruled to be accidental.
We see an example of this in Harry Potter and the Deathly Hallows. In this instance, the total lack of marks on the family lead the authorities to suspect that they died as a result of some manner of gas inhalation.
‘Meanwhile, in Gaddley, a Muggle family of five has been found dead in their home. Muggle authorities are attributing the deaths to a gas leak, but members of the Order of the Phoenix inform me that it was the Killing Curse – more evidence, as if it were needed, of the fact that Muggle slaughter is becoming little more than a recreational sport under the new regime.
Harry Potter and The Deathly Hallows, p. 439, Scholastic, Chapter 22 :The Deathly Hallows
