Police believe the female witness engaged in a conversation with the mother who at the time was stranded in a broken down Ford Falcon at the intersection of Osprey Drive and Dotterel Way in Yangebup.
Police found the baby dead at his mother’s home the following morning.
It is thought the conversation took place sometime between 9.30pm, January 30, and midnight, January 31.
The mother, in her early 20s, was waiting while a male friend went to retrieve fuel. Police believe her young son was in a capsule in the rear of the blue station wagon.
Detective Sergeant Matt Garnaut hoped the witness could shed some light on the events of that night as police worked to piece together how the baby died.
‘This witness may have seen the baby in the rear of the car and may be able to give a clearer picture of the baby’s condition and how the baby was seated in the vehicle,’ he said.
Police were notified of the infant’s death on January 31.
Anyone with information should call Crime Stoppers on 1800 333 000.