The viability of a police theory William Tyrrell's foster mother loaded his body into a car and disposed of it close to where he died will be examined at a reopened inquest into the toddler's disappearance.
The three-year-old went missing while playing on the verandah at his foster grandmother's home in Kendall, on the NSW mid-north coast, on September 12, 2014.
On Monday, counsel assisting Gerard Craddock SC reopened the inquest for a fifth round of evidence, saying the focus would be on a theory the foster mother was involved in unlawfully disposing of William's body after his accidental death.
After the three-year-old died at the Kendall property, police believe the foster mum might have loaded his body into her mother's Mazda before driving down the road to dispose of his remains, deputy NSW coroner Harriet Grahame heard.
It was only then that she called triple zero, according to investigators' theory.
"Police assert that she must have quickly resolved that if the accidental death were to be discovered, she might lose (her other foster child)," Mr Craddock said.
The foster mother denies having anything to do with William's disappearance.
Evidence will be given from a truck driver who drove past where the foster mother allegedly disposed of the body at the time.
The inquest into William's disappearance began in March 2019 but was adjourned in October 2020.
No one has been charged and a $1 million reward for information stands.
Despite a decade-long investigation involving hundreds of persons of interest and dozens of searches, no trace of the boy - last seen wearing a Spider-Man suit - has been found.
This round of the inquest will examine a 2021 search of the Kendall property and surrounds - the third by police into William's whereabouts.
Mr Craddock said after an initial search for the toddler in September 2014, further sweeps of the area using GPS trackers and police dogs were conducted in 2018 and 2021.
"The 2018 forensic search was an intensive, thorough search," he said.
"By comparison, the 2021 search was at a further level of intensity."
The court heard from Griffith University's Professor Jon Olley, an expert in geomorphology and how water changes landscapes, who helped NSW Police design and carry out the search.
He said officers canvassed the area in a "very thorough" manner.
The polyester Spider-Man suit William was wearing would have lasted hundreds of years if the three-year-old's body had been dumped in a local creek, Prof Olley said.
Rubber-soled shoes and sandals would have lasted for decades before breaking down, he added.
In looking for William, police followed through on tip-offs from prison inmates and clairvoyants claiming to have information, the court was told.
"Given the circumstances of possible pedophile involvement in William's disappearance, it would have been remiss of police not to follow up on these leads," Mr Craddock said.
One lead involved neighbours hearing a car doing a U-turn in the street outside the foster grandmother's home about the time of the disappearance, he told the court.
But police were unable to verify that information or track down the vehicle.
Mr Craddock reminded the coroner she did not have the luxury of "gut feel" when it came to determining what happened to William.
"Guesses are not rational, suspicions form no rational basis for making findings of fact," he said.
Pedophile Frank Abbott - who was a person of interest in William's disappearance but cleared by police - watched the inquest from a cell at Long Bay prison telling Ms Grahame he wanted to give further evidence.
Other potential suspects such as Bill Spedding and Paul Savage were investigated but later cleared of involvement in the kidnapping, the coroner heard.
Mr Spedding successfully sued the State of NSW for damages after being maliciously prosecuted by police officers.
The inquest continues on Tuesday.
