Frank Mennilli, now an assistant commissioner and commander of the South-West Metro Region, was the lead investigator in the disappearance of Richard Sajko, who vanished the night before Mother's Day in 1995.

Mr Sajko, a "quiet and shy" 21 year-old, was last seen leaving his job at Avis Car Rentals in Mascot just days before he was due to give evidence against a former friend in a criminal trial.

More than two decades later, no one has been arrested despite revelations on Tuesday that a prime suspect had confessed.

His mother Rozi Sajko told an inquest at Glebe Coroners Court that her son seemed "very disturbed" in the days before his disappearance.

She had learnt two weeks earlier that he was defending criminal charges in a district court after being pulled over in October, 1994, in a tow truck being driven by his friend Sam Testalamuta with a stolen car on the back.

The pair had given police conflicting explanations of how the stolen car came to be in their possession and Mr Sajko revealed to his mother that Mr Testalamuta had been pressuring him to change his statement.

At one stage, Mr Testalamuta threatened to kill Mr Sajko and blow up his car, the inquest heard. He had also been using Richard's name and address, without his permission, to get out of traffic infringements.

"He seemed disturbed," Mrs Sajko told the inquest. "Even though he was reserved and a shy person, with me and in our home he just was a normal, happy person but he didn't seem [that way] at the time. When I asked him what is the matter, he came out with the story... about the trouble that he seemed to be having with Sam Testalamuta."

He was due to spend Mother's Day with his mother at her Coogee home in 1995 but never showed up.

The night before, he left work at midnight with a passenger in his car. His red Commodore was parked oddly in a Croydon street that night, where it remained for 11 days until police began to investigate.

In the early stages, Mr Mennilli told Mrs Sajko that her son had probably made up the story about being threatened and simply run away, the inquest heard.

"Did Sergeant Mennilli ever tell you that he thought there was no evidence in this case supporting foul play?" asked Penelope Wass, SC, representing the Sajko family.

"Yes," Mrs Sajko replied.

She said she would often ring Ashfield police to ask questions and would never receive a call back.

It was only two years ago that she was told a man, John Tuiletufuga, who has since been deported to New Zealand due to criminal convictions, had confessed.

There was evidence that Tuiletufuga and another man known only as Sam had used Richard's phone the day after he disappeared.

One of several issues to be addressed in the inquest is whether serious errors by the police allowed Mr Sajko's unusual and suspicious disappearance to go unsolved for almost two decades and possibly allowed a murderer to remain free.

"There is no evidence that Richard ran away or committed suicide or died from drugs," counsel assisting the coroner, Ian Bourke, said. "The evidence does support the conclusion that Richard has died and died in very suspicious circumstances."

Mr Sajko's body has never been found.

The inquest continues.