The toddler - who was last seen wearing a Spider-Man costume - 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.
An inquest into his disappearance was reopened on Monday for further evidence relating to police searches into his whereabouts and a theory that his foster mother disposed of his body after a fatal accident on the property.
The woman, who has denied anything to do with William's disappearance, allegedly hid the body out of fear another child under her care would be taken away, Lidcombe Coroners Court previously was told.
As the inquest continues on Wednesday, a truck driver who passed through the neighbourhood on the day the toddler vanished is expected to give evidence of what he did at the time and who he encountered.
The inquest will then view two electronically recorded interviews by police of another key witness, who cannot be named or identified, in 2014 and again in 2022.
This will occur in a closed court session.
Footage of the foster mother giving evidence before the NSW Crime Commission will also be played before the inquest comes to a close.
No one has been charged over William's disappearance 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 has been found.
