William Tyrrell was dressed as Spiderman, climbing trees and "pretending to be a daddy tiger" on the morning of his disappearance in 2014, a NSW coroner has been told.
"We heard him roaring around the garden and then I thought, oh I haven't heard him, I better go check on him, and couldn't find him," his foster mother told a triple-0 operator on September 12.
She said the three-year-old boy was wearing a Spiderman suit and she'd been searching the neighbouring properties and green bushland for any sign of red.
The woman also told the operator she hadn't seen anyone suspicious in the area.
The audio file was played on Monday at the first day of a Sydney inquest into William's disappearance and suspected death in the "sleepy village" of Kendall, on NSW's mid north coast.
Counsel assisting the coroner, Gerard Craddock SC, said William's foster grandmother - whose house the family were visiting - had noticed "it had become quiet, too quiet".
"There had been one loud roar and then nothing," Mr Craddock said.
In a police video played to the inquest, from the backyard of the property, William's foster father told an officer: "He never wanders. He's not a wanderer."
"I was thinking so many things all at once. I need to cover as much ground as I possibly can in the shortest amount of time," the man, who cannot be identified, said.
The first week of hearings will explore William's foster and biological families, when he disappeared and early stages of the investigation including the action taken shortly after he went missing.
Further hearings will begin in August when persons of interest will be called to give evidence.
The inquest continues before Deputy State Coroner Harriet Grahame.
William Tyrrell coronial inquest
DOZENS of people, several who have been identified as persons of interest, will take the stand as the coronial inquest into the disappearance of William Tyrrell begins.
In September, it will be five years since the then toddler wearing a Spiderman suit vanished without a trace from his grandmother's front yard in Benaroon Drive, Kendall.
A search that has not wavered since that time has found little evidence, but investigators leading the strike force to find answers believe the truth rests with the predator who abducted him.
This year the thousands of pieces of evidence and hundreds of interviews with witnesses and persons of interest have built the case that William was taken by someone who now holds a very closely guarded secret that needs to be revealed.
All of the evidence collected by Strike Force Rosann, led by head investigator Detective Chief Inspector Gary Jubelin, will be presented before the NSW Coroner's Court from today (March 25).
Witnesses will take the stand in Sydney in what is the next big move in the case following on from a large scale forensic search last year back where it all began in Kendall in September 2014.
"Co-ordinating and documenting the areas we search and the purpose of this evidence, whether it's presented to a court - coroner or criminal - is to prove that beyond reasonable doubt, William's disappearance was the result of human intervention and not misadventure," Det Ch Insp Jubelin said at the time.
As we embark on this next phase of our search for William, we offer our deepest gratitude to the many who have supported us over the past four years, six months and 13 days.William's family.
At a directions hearing for the inquest in December, counsel assisting the coroner Gerard Craddock SC said it wasn't possible - yet - to conclude William was dead.
The first week of hearings will see William's family and loved-ones "set the stage" for investigators to show the three-year-old was likely snatched by a predator, a police source told AAP this month.
"Not a day goes by that William is not in our hearts and in our thoughts - the grief is overwhelming," the family shared through the official Where's William support page.
"On Monday, March 25 we will mark 1655 tragic days since our beautiful little boy went missing, and with it comes renewed hope that the coronial inquest into William's abduction will enable Strike Force Rosann to test the information and evidence they've painstakingly gathered to help further their investigation to find out what has happened to our precious little boy.
"As we embark on this next phase of our search for William, we offer our deepest gratitude to the many who have supported us over the past four years, six months and 13 days.
"To those who combed bushland from daylight til dark; the media for keeping William's disappearance in the public eye; the Where's William? volunteers who helped spread the word that William is still missing; the thousands of people who walked and those who shared William's image across the nation and around the globe; the 105,000 followers of William's Facebook page who keep our message in the hearts and minds of all Australians."
"We thank Crime Stoppers NSW and the NSW Police Force for their undying commitment; but above all, Detective Chief Inspector Gary Jubelin and Strike Force Rosann whose dedication to the search for William is unparalleled - our heartfelt thanks to you all."
Throughout the investigation numerous persons of interest have been identified. Some were charged with unrelated crimes as detectives dug into their pasts, others were cleared entirely.
In 2015 it emerged a ring of paedophiles had been active in the area and were being investigated. Years later, that theory has not conclusively been ruled out.
William and his younger sister were in foster care when he disappeared.
A second round of hearings, beginning August 5, will be watched closely by detectives as people they've identified as worthy of suspicion take the stand. Under the powers granted to the coroner, these persons of interest may be forced to explain their movements and knowledge of William's disappearance before the court.
A $1 million reward for information leading to return or recovery of William still stands. It is one of the biggest rewards on offer in NSW.
Anyone with information should call Crime Stoppers on 1800 333 000.
Port Macquarie News with AAP