Jake Lyons, 21, described as altruistic and intelligent by those his disappearance has tormented most, has not been seen since leaving his family's Dandenong North home on August 25.

He has little in common with Karen Rae, a 48-year-old Frankston mother of four, except both are still missing.

Fairfax Media can reveal there have been breakthroughs in both cases, including a potential sighting of Ms Rae the day after she vanished, nearly four months ago.

These developments come on the eve of national missing persons week, as investigators move to dispel common myths, including that choosing to vanish is a crime and that you must wait 24 hours to report somebody missing.

Half of the 9245 people reported missing in Victoria last year were located within 24 hours and about 75 per cent were found within the first three days.

By the end of 2014, almost 98 per cent of those cases had been solved, leaving just 123 people whose fate remained a mystery, the bulk of whom were children and teenagers.

In December, Mr Lyons' case was handed to the cold case missing persons squad after police received specific intelligence suggesting he had met with foul play.  But that information has now been discounted.

His father's car turned up at a reserve in Springvale with the keys carefully tucked underneath and no signs of a struggle within.

"One day I'm thinking is he alive, I really don't even know if he's alive or is he just living off the streets, it changes all the time." cousin Kim Brassington said.

"It's heartbreaking, you're always looking for them and if I drive along the freeway I think, is he in the bushes somewhere?"

Before he vanished, Mr Lyons had spoken of wanting to join the military, of starting a new life in the outback and also of emotional struggles, scattering what would later become clues in the hands of detectives.

Detective Senior Constable Kane Taylor, from the cold case and missing persons squad, said he believes Mr Lyons chose to walk off on his life and it was possible he could be living in the outback.

But he has not ruled out the possibility Mr Lyons met with foul play later or took his own life.

Extensive sweeps of Dandenong and the reserve where the car was found, including dredging nearby catchments, have failed to locate any trace of him.

To deliberately disappear would require significant planning, stealing somebody else's identity would set off alarm bells and changing a name would leave an electronic paper trail.

"Anyone's capable of it, but it's not something you would just wake up one day and decide to do," Senior Constable Taylor said.

In Karen Rae's case, a potential witness has come forward and provided police with details to form a sketch of an unknown man possibly seen with her the day after she vanished on April 15.

Ms Rae may have been driven to the Seaford Hotel by a male friend the night she vanished but she was not captured on any closed circuit television footage at that hotel, nor the nearby Sands, that night.

"She had friends she would meet up with there and other associates, we're hoping that from the 15th of April somebody may have seen her," Detective Leading Senior Constable Simon Florence said.

Investigations have centred on whether Ms Rae has fallen victim to violence or chosen to leave.

Her mother, Christina Boyle, said: "It's something that is never too far away from your thoughts. I could be going about a normal day and just one thing reminds me of Karen and I'm immediately frozen. It's not having an answer that is the hardest – has something happened to her? Did she decide to leave? Is she alone?"

"She's my daughter. I love her. I just want to know that she's ok."

Ms Boyle said it was difficult to explain her grandsons what had happened to their mother.

"I regularly see her 10-year-old son and it is heartbreaking trying to explain to him what's happened," Ms Boyle said.

"He's very sad and misses his mother. He asks to see photos and talk about what she was like when she was his age.

Falsehoods about missing persons, including the need to wait 24 hours, are the result of American crime dramas, said Rebecca Kotz, the head of the Australian Federal Police's national missing persons centre.

"If somebody you know is not where they should be and you've got that gut feeling that there's something wrong, go to the police, don't wait 24 hours," she said.

"That first 24 hours is critical, that first 24 hours could be the difference between finding people alive and dead."

If you believe you have information on the whereabouts of Ms Rae or Mr Lyons please contact Crime Stoppers on 1800 333 000.

with Deborah Gough