Three years and 1000 calls to Crimestoppers on, police are still waiting for the one tip-off to help them find missing Melbourne schoolgirl Siriyakorn “Bung” Siriboon.

Police have chased down a mountain of leads, doorknocked 1000 homes, interviewed 250 registered sex offenders and offered a $1 million reward.

Homicide squad Detective Inspector John Potter said it might take a tip-off to find Bung, who disappeared on her walk to school in Boronia in Melbourne’s east.

“Someone may tell someone else what’s happened to Bung and that’s the person that we need to come forward,” Det Insp Potter told reporters on Monday.

“We continue to hold grave fears for Bung.”

What’s for sure is Bung’s neighbour saw her walk along Elsie St shortly after 8.30am on June 2, 2011.

Beyond that the details are sketchy.

Police thought Bung made it all the way along Harcourt Rd to Moncoe St, just 130 metres from her school.

But another report, that a driver saw a teenage Asian girl – possibly in a school uniform – in the back seat of a white Ford station wagon on Boronia Rd, puts that in doubt.

“They were driving, they were stationary at the traffic lights, they saw this happen, they felt there was something odd about what they were seeing, so they reported it,” Det Insp Potter said.

The driver, aged in his late 50s or early 60s, had a tattooed left arm and a “rock and roll styled hairdo”.

“If this is a perfectly innocent scenario we need to find out,” Det Insp Potter said.

Police had held onto the information about the sighting in the car “for some time” until they were sure it was on the day Bung went missing.

He said police had spoken to persons of interest, but none of them owned a white station wagon.

In 2013 a man claimed he hit Bung with his car and dumped her body at a local reserve.

But no remains were found, and police said parts of his story didn’t add up.

Det Insp Potter said police “always hold out hope” Bung may be found alive.

 AAP