The 3-year-old was playing with his grandmother in the yard of their Kendall, NSW, home, when he mysteriously vanished on September 12, 2014.
Investigators have now discovered a burnt-out car that was left deep in the bush near where William disappeared.
It is believed the vehicle belonged to Tony Jones, who was at one point a person of interest in Tyrrell's disappearance.
A Current Affair previously reported that Tony Jones had denied any involvement in the child's disappearance.
Police were informed of the vehicle hidden in bushland, but once they arrived at the scene they found the car had been flipped over and set alight.
A woman Mr Jones was believed to be dating at the time of William's disappearance claimed she 'wouldn't be surprised' if it was his car.
William has not been sighted since his disappearance, despite a large-scale search and one of the biggest manhunts in Australia's history.
On the fourth anniversary of William's disappearance, police announced they would be requesting an inquest, Deputy NSW Coroner Harriet Grahame said.
It is expected to be held next year and could be the last hope of piecing together who was responsible for the boy's abduction.
A statement from NSW police said, 'Over the past year, investigators have continued to explore lines of inquiry in an effort to find out what happened to William, including a large-scale forensic search.'
'William's families have been informed of this development and have requested their privacy at this time.'
Also on the fourth anniversary , William's family wrote to Facebook: 'Today marks four tragic years since we last saw little William. Four years of heartache and four years of endless tears for everyone who held him dear - not knowing where he is or what has happened to him,' the post read.
'Today also marks the gift of unending love, hope and support from across Australia and around the globe.
'To all those who have cared; to all those who have supported William's loved ones in our quest to find him... our sincerest thanks for not giving up on our boy... you have our complete trust and our deepest gratitude for your continued commitment to finding out what has happened to William. Our heartfelt thanks to you all.'
This article originally appeared on New Idea