Twenty-three- year-old 'loner' Ian Stanton was reported missing by his parents 18 days ago after they visited his Bundanoon flat to find he had vanished without a trace.

Despite a wide-ranging investigation, including door knocks and a trail bike search through bushland, police remain baffled by the case.

The unemployed man was a known drifter and avid bushwalker while police believe he may have had a link with a strict religious group.

"We've had no response at all from anyone and even the Bundanoon locals are a bit vague," head of the investigation Senior Constable Kevin McCudden said.

"It's a real mystery and it's extremely frustrating.

"Again we would urge anyone who has any information, no matter how small, to contact police or the missing persons unit."

The trail bike squad scoured bushland in the Morton National Park last Friday and Saturday- a place where Mr Stanton was known to frequent.

An air search of the area has been ruled out at this stage.

Mr Stanton disappeared with just the clothes on his back and left both his wallet and keys behind.

"He may have got on a train, gone for a bushwalk or even hitchhiked up north, which he was known to do," Sen Con McCudden said.

"But if he was going away you'd expect him to pack something and there's a chance he didn't want anyone to know what he was doing."

Police have also had hours of railway video surveillance studied to see if Mr Stanton boarded a train.

Mr Stanton is described as 190cm tall with a medium build and light brown shoulder-length hair and a full beard.

Police suspect he was wearing predominantly grey at the time of his disappearance.