Australian Federal Police were on the scene on Thursday with ground penetrating radar examining the slab, however at this stage no inconsistencies appear to have been detected.
Meanwhile in nearby bushland, plastic sheets have been laid down in areas where police were digging, in anticipation of rain expected to hit Kendall this afternoon and Friday.
Earlier, Prime Minister Scott Morrison praised police officers investigating the three-year-old's disappearance, saying they had made "huge inroads".
Mr Morrison said police would "never stop" looking into the case, and that it was a great statement about the dedication of police.
"My late father was a policeman so I have a bit of an inkling about the level of patience and determination required from law enforcement to never give up," Mr Morrison told reporters on Thursday.
NSW Police revealed on Wednesday they'd seized a car that belonged to the foster grandmother, who has since died.
The grey Mazda was taken from a home in Gymea in Sydney's south under a coronial order last week and is undergoing extensive forensic examination.
Also on Wednesday, William's foster parents were charged with the unrelated assault of a different child and will face Hornsby Local Court on Tuesday.
The former homicide detective who was initially in charge of the case on Thursday defended his handling of the investigation.
Retired homicide detective Gary Jubelin was removed from the case after four years.
Police Commissioner Mick Fuller this week said the new investigation team had "inherited what was a bit of a mess".
But Mr Jubelin said he provided monthly progress reports to his superior officers detailing everything - "what suspects I was targeting, what the future directions were".
Mr Jubelin admitted he had formed a friendship with William's foster parents and believed the foster mother to be "a very decent human being".
However, he said he "went hard" when investigating the couple.
Mr Jubelin eliminated them as suspects after a covert operation that included placing a listening device in their car.
The investigation is now considering whether William might have died after falling from the balcony of the foster grandmother's house.
Some 30 to 40 people are helping with the search, including officers from NSW Police and the Australian Federal Police, as well as Rural Fire Service volunteers.
The findings of a coronial inquest into William's disappearance, which concluded last year, are yet to be handed down.
A $1 million reward for information on the case still stands.
10°C