An expert believes four-year-old Gus Lamont, who went missing one month ago in the South Australian outback, could have travelled beyond the search zone.

The little boy was last seen playing on a mound of dirt on his grandparents' remote property near Yunta, in the state's Mid North, on September 27.

August 'Gus' Lamont was under the care of his grandmother Shannon Murray as his mother Jessica and grandparent Josie tended to their herd of sheep 10km away.

Police conducted two separate searches, one straight after Gus disappeared, and then another last-ditch effort launched on October 14.

More than 100 members of South Australia Police, the defence force, and the SES have failed to find any trace of Gus apart from a single footprint in the desert scrub.

An expert in human physiology from Flinders University has offered an alarming new theory that the four-year-old may have moved outside the search area.

'Over a three-day period we're looking at potentially three to eight kilometres,' Nina Siversten told 7NEWS on Monday.

The initial search radius was originally 2km surrounding the homestead. The second effort extended to an extra 2.5km to 3km further into the scrub.

August 'Gus' Lamont vanished on September 27 after playing in the yard of remote Oak Park homestead in South Australian outback, nearly 200 miles north of Adelaide

More than 100 members of South Australia Police, the defence force, and the SES failed to find any trace of him during two separate searches

Ms Siversten also highlighted the minor but crucial factors that may have allowed Gus to survive longer than three days in the harsh bushland.

'If the child could access some sort of moisture or dew or moist leaves that could increase it (survival) somewhat beyond the three days,' she said.

'I think that fear would be an absolute factor and that would impact on the ability to move but also on finding shelter.'

Former SES volunteer Jason O'Connell had walked the property alongside Gus's father Joshua Lamont and used his tracking skills during the search.

'I just don't get how (Gus) vanished like that,' he said.

'It's been searched. (Gus) is not there.'

He said his heart breaks for Mr Lamont, a country music singer who toured South Australian pubs with his band The Cut Snakes.

Despite the sheer size of the Murray property, thousands of online commenters have questioned just how a little boy could simply vanish, especially after SAPOL deployed every available resource at recovering him, alive or otherwise.

Gus went missing form his family's sprawling sheep property in the middle of nowhere in outback South Australia, nearly 200 miles north of Adelaide

Human physiology expert Nina Siversten (pictured) has warned that Gus could have travelled outside the search zone by moving three to eight kilometres over a three-day period

This included an Indigenous tracker, infrared drones, an AirPol helicopter and sniffer dogs.

Still, the only sign of Gus was a single footprint in the dirt about 500metes from the family homestead - and police have since said it may not be relevant to their investigation.

Hours of drone footage taken at the property is currently being reviewed and no future searches are planned.

Police have insisted there was no sign of foul play, and that the devastated family has been assisting with their investigation.

The investigation into Gus' disappearance has been referred to the Missing Persons Unit, and will remain open.

Gus has long, blond, curly hair and was last seen wearing a grey sun hat, along with a blue T-shirt with a yellow Minion on the front, light-grey long pants and boots.

Expert shares chilling new theory about missing boy Gus Lamont