It is understood computer equipment and a single mattress was seized from a unit in Laurieton, south of Port Macquarie, searched as part of the ongoing investigation into the little boy's disappearance four months ago.

The items are expected to undergo urgent forensic examination as police continue their inquiries.

Police sources have told Fairfax Media it is most likely the three-year-old was abducted.

"We don't think he has just wandered off somewhere," a senior officer said.

William vanished without a trace from his grandmother's Kendall home, just south of Port Macquarie on September 12.

He was wearing a Spiderman suit and playing in backyard with his sister when he disappeared, as their mother made a cup of tea.

On Tuesday, detectives from Sydney travelled to the neighbouring suburb town of Laurieton to search a unit above a set of shops.

Up to a dozen detectives and forensic officers spent several hours searching the unit on the corner of Bold and Seymour Street.

A local employee who works in a shop beneath the units said the police spent most the day searching the premises.

"They've been here all day from what we could tell – just forensic people here all day," the man, who did not wished to be named, said.

"They left this afternoon and took a whole lot of things with them," he said.

"The detectives have been in and out all day, up and down the stairs."

The man said he believed a man and woman lived in a unit above the set of shops but said he had never seen anyone in the office space where police had focused their investigations.

"They were looking through the office."

It is understood a number of people were interviewed on Tuesday as part of the ongoing investigation into the toddler's disappearance.

"Today police are searching a number or premises and speaking with numerous people as the search for William Tyrell continues," a NSW Police spokesman said.

Fairfax media understands homicide detectives, along with local police from the mid north coast have searched a number of properties in recent months.

A 10-kilometre search of the area surrounding the house failed to find any trace of William.

The search lasted weeks and involved the NSW Police, Rural Fire Srvice, the State Emergency service and more than 200 locals.

Investigators have searched every corner of the 21 houses in the bushland estate where William was last seen.

Commander of the mid-north coast region Superintendent Paul Fehon has recently said police were will looking at a number of scenarios.

Superintendent Fehon said that, if the little boy had some form of misadventure in nearby bushland, police would have found something by now.

"We are completely open to any possibility, including human intervention," he said in an interview with Fairfax Media.

"If that has occurred, somebody knows something."

He said police have sifted through hundreds of pieces of information that poured into Crime Stoppers, but still have no solid lead.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.