William Tyrrell's foster mother is being treated as a person of interest in his disappearance as the search for the boy's remains continues around the NSW property, he was last seen in 2014.

It comes as a car belonging to William's now-deceased foster grandmother at the time he went missing was also seized from a home at Gymea, in Sydney's south, under a coronial order last Tuesday.

The vehicle, a grey Mazda hatchback, is undergoing forensic analysis, which is expected to take several weeks. The car has changed ownership since the woman died in March. "This activity relates to inquiries and search operations currently being conducted in the Kendall area," NSW Police said in a statement.

William, at the time aged three, disappeared from the foster grandmother's property on Benaroon Drive in Kendall at about 10.30am on September ...