She also provided a detail description of a man seen in Benaroon Drive, Kendall around the time her little boy vanished.

"My heart just sank because I just thought those two cars were there for both of them," she said on Monday, referring to the three-year-old boy and his older sister.

The woman told day one of the Sydney inquest into William's disappearance and suspected death she recalled the white and gunmetal grey vehicles with tinted windows in the days after he went missing from Kendall in September 2014.

Detective Chief Inspector Gary Jubelin, heading Strike Force Rosann, made public the suspicions the investigation team had about the cars almost one year after William disappeared.

"The reason we want to find out details of those two vehicles is the manner in which they were parked," Det Ch Insp Jubelin said at the one year anniversary of William's disappearance.

He said it was a "tactical decision" to release the information about the vehicles almost a year since the incident.

"They were parked one behind each other but it was a dead end street and they were parked between the driveways so that causes suspicion as to what they were doing there."

Counsel assisting the coroner, Gerard Craddock SC, asked William's foster mother why the cars weren't mentioned in her original statement.

"I didn't even think about it because I saw the cars, walked back inside and got swept up in the emotions of getting everybody ready for the day and then with William missing it went right out of my brain," she replied.

Officers have repeatedly quizzed local residents about visitors to the secluded neighbourhood and have ascertained that two vehicles were parked by the roadside opposite the property then owned by William's grandmother on the day he disappeared.

A car from Wauchope, owned by a local man who was sentenced around the same time to three years in jail after he was convicted of the aggravated indecent assault of a young child, was seized and examined by the Strike Force.

William's foster mother said she could still picture a man she stared at that morning driving a third, green-teal-looking car which then reversed in a driveway and drove off.

She said he was "a big man" in his late 50s who was Caucasian, had "sandy, reddy-coloured hair", a "thick neck" and looked "weathered".

She said she was still working with police to reconstruct the man's appearance with software but had a "pretty intense reaction" when she identified the green car.

In his opening address this morning, Mr Craddock said he expected the evidence before the inquest would show it was likely William "was taken".

"That is, that William's disappearance was the direct result of human intervention," he said.

"If the evidence establishes that William was abducted, that conclusion is chilling because it means a person snatched a three-year-old from the safety of a quiet village backyard."

In a police video aired in court, taken in a backyard on September 18, William's foster father tells an officer: "He never wanders. He's not a wanderer."

A triple-zero call made by the foster mother on September 12 was also played.

"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," the woman says.

The boy was wearing a red Spiderman suit and had been roaring like a "daddy tiger" that morning.

The foster mother had searched the neighbouring properties and green bushland for any sign of red, and estimated William had disappeared about 10.30am.

She also told the operator she hadn't seen anyone suspicious in the area.

The first week of hearings will explore William's foster and biological families, when he disappeared and the action taken shortly after he went missing.

Mr Craddock said there is "no doubt" William's biological parents were in Sydney the day he disappeared.

"Investigators haven't positively drawn the conclusion that no relative or associate was involved in William's disappearance," he said.

Further hearings, also before Deputy State Coroner Harriet Grahame, will be held in August when persons of interest will be called to testify.

"I acknowledge at the outset, to have a child go missing must be one of the greatest pains a human can experience," she said on Monday.

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 pedophiles had been active in the area and were being investigated. Years later, that theory has not conclusively been ruled out.

Under the powers granted to the coroner, persons of interest may be forced to explain their movements and knowledge of William's disappearance before the court.