Officers believe the woman came to the aid of a man who broke down somewhere in Melbourne's northern suburbs while driving a dark green VT or VN Commodore car.
It is thought she may have picked the man up between 12 noon and 3pm on Thursday, taking him to a service station nearby.
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Gurshan Singh Channa disappeared from his family's home in Lalor on Thursday.
The three-year-old's body was found in grass by the side of a road some 20km away, at about 7pm that night.
Gursewak Dhillon, 23, has been charged with the boy's manslaughter by criminal negligence.
He appeared briefly in Melbourne Magistrates' Court on Tuesday.
Dhillon, a part-time taxi driver and housemate of the toddler's parents, will appear again in court on June 29.
