William Tyrrell's foster mother has cried while telling a NSW coroner of the moment she recalled seeing two cars parked on a nearby road the morning he disappeared.
"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 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 in September 2014.
The family had been visiting his foster grandmother's house in Kendall on the NSW mid north coast.
Counsel assisting the coroner, Gerard Craddock SC, asked 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.
The woman also wept when she recalled seeing a third vehicle that morning - a green or teal-looking car which reversed in a neighbouring driveway and drove off.
She said the driver was "a big man" in his late 50s who was Caucasian, had "sandy, reddy-coloured hair", a "thick neck" and looked "weathered".
They exchanged a fleeting "Why are you watching me? I'm watching you" glance, she said.
The foster mother 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 "old" green car.
"I can't tell you how much I rack my brain and I beat myself up over not looking at number plates," she said.
She also recalled the exact moment when William went missing while playing "daddy tiger" and running out of sight.
"I hear a roar and then I hear nothing," she told the court.
She raced around the property, looking in every cupboard and every bush for a sign of his red Spiderman suit.
"William, it's Mummy. You need to tell me where you are. You need to say something," the woman recalled through tears.
"He was gone."
In his opening address, Mr Craddock said he expected the evidence before the inquest would show it was likely the boy was taken.
"That is, that William's disappearance was the direct result of human intervention," he said.
"If the evidence establishes 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 six days after William vanished on September 12, his foster father tells an officer: "He never wanders. He's not a wanderer."
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 was 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.