The three-year-old, dressed in a Spider-Man outfit, vanished while playing in the garden of his foster grandmother's home in the small mid-north coast town of Kendall on September 12, 2014.

During a thorough search in 2018 of undulating, dense bush near the house, police came across a cross with the year 2014 written on it.

"It struck my curiosity," Senior Constable Grant Hollis told the NSW Coroners Court on Thursday.

The experienced searcher - who described combing the forest as the largest operation he'd been involved with - said detectives and officers with shovels were quickly called to the grave.

"Digging it up was not like how you dig at the beach. It was, dig an inch, dig an inch," he said.

"(Then) there was blue plastic, so we've carefully dug around the blue plastic to completely uncover it.

"It was about a foot long and half a foot wide and we discovered (it contained) the corpse of a dog - a small dog."

Dozens of police, SES personnel and other support agencies spent four weeks in 2018 combing more than 40 hectares of the forest.

The searchers were told to flag anything that appeared out of place. They posted photos to a WhatsApp group and called the control post for possible review.

A white baseball cap, a rusty crowbar, a Monopoly set with a notebook and a car's service log book were among other items bagged as possible evidence, the inquest was told.

"We leave the decision to investigators whether it is related or not," Snr Const Hollis said.

"The majority was just rubbish, chip packets and your general household waste, clothes, items off motor vehicles, number plates, a service book which turned out to be from a stolen vehicle.

"The search was not only of the ground, if we got to a cliff or a tree, we'd look up or down."

The Tyrrell inquest resumed this week for a second round of hearings examining the fresh police investigation into William's disappearance.

All of the evidence collected by Strike Force Rosann is being presented before Deputy State Coroner Harriet Grahame who will determine if the three-year-old did succumbed to the rugged bushland around his grandmother's home, or was abducted by someone. There is evidence yet to conclude that William is dead.

During his opening address on Wednesday, counsel assisting the coroner Gerald Craddock said William was likely taken from the area in a car.

He described the person who took the three-year-old as "a sneaky, complex offender who has hidden their desires for some time".

The inquest was told police weren't aware of any eyewitness or forensic evidence - making it among the toughest cases to solve. But detectives remain hopeful.

A $1 million reward remains in place for information leading police to the whereabouts of William.

The coronial inquest will sit for two weeks before travelling to the Taree Courthouse on August 19 where theories about what may have happened and who may have been involved will be examined.

Jailed people and police detectives would be among about 54 witnesses to give evidence during the hearings in Sydney and Taree until August 30, Mr Craddock said.

He stressed any suggestion that those called to give evidence were suspects was "simply wrong".

"This is an inquest and not a criminal trial," he said.

"There has not yet been a conclusive breakthrough (in the police investigation), otherwise someone would have been charged and we wouldn't be here."

Australian Associated Press