The third episode of true-crime podcast Annette: Cold Case Unlocked examines the police investigations that have been conducted into teenager Annette Deverell's disappearance and death almost 40 years ago.

Someone Knows Something looks at the missing persons inquiries conducted immediately after Annette disappeared, the police investigation following the discovery of her remains in nearby bushland two years later and the review of the case in 1999.

Questions are raised about items related to the case being held by police and a startling piece of evidence, which has the potential to be tested for DNA, is uncovered.

A serial killer theory, suggested by investigators and Annette's friends, is examined, as are suspicions that a notorious Perth couple convicted over other killings might have murdered Annette.

Finally, we take a closer look at the panel van in which Annette was last seen.

Retired detective Jeff Beaman, who led the 1999 review of the case, shares the struggles he faced, from a questionable police culture to an unusual file management system and a high-profile series of murders that took the police focus at the time.

Mr Beaman said he wished he had solved the case.

"It is one of those things that sticks in an investigator's mind who couldn't solve something. I feel it now - that I didn't," he said.

His case review faced many obstacles.

"I knew my days were numbered because you clash with people who were supposed to be the go-to people," he said.

Episode one, The Skeleton in the Forest, explores the night Annette Deverell, then 19, disappeared from outside a post office in Mandurah, south of Perth, on a cold and windy Saturday night.

She was given a lift into town to buy cigarettes by her mother, Margaret Carver.

Annette said she would get a lift home with her friends.

But she never returned.

Her remains were discovered two years later in bush about a 30-minute drive from where she was last seen in Mandurah.

Mrs Carver has called for justice after 39 agonising years of unanswered questions, with no reward for information or coronial inquest into her daughter's death.

She believes Annette knew her killer.

Episode two, Who was Annette Deverell?, revealed long-held secrets and suspicions as Annette's friends and acquaintances recalled their memories growing up in the sleepy seaside town.

The third episode, Someone Knows Something, also hears from former police detective Bruce Scott, who worked in the region where Annette lived and later assisted with her case.

He provides a valuable insight into crime in the area in the lead up to her disappearance.

Mr Scott was one of two detectives investigating serious crime in an area spanning almost 1500 kilometres.

He recalls it as the "busiest Criminal Investigations Branch in the state".

These days, more than 40 detectives cover that same region.

It is telling that when Annette disappeared from Mandurah, there was not a single detective based in the town.

This meant that the missing person investigation was led by detective Barrie Rolinson, based in Midland, about 90 kilometres north of Mandurah.

Someone Knows Something details the struggles the late Mr Rolinson faced when Annette went missing and his initial suspicion that something more sinister had happened to the teenager.

He knew she hadn't run away from home - she was close to her family and friends and had no reason to.

Annette's friends share the details of their involvement with police after her disappearance and again after the discovery of her remains.

Many of those friends say both investigations tapered off too quickly.

If you have any information that could help police solve this case, contact Crime Stoppers on 1800 333 000. You can contact Carla Hildebrandt by emailing annettepodcast@gmail.com.

Photo credit: Margaret Carver, Mandurah Historical Society, Carla Hildebrandt.

Music credit: https://www.freesfx.co.uk.