The grandmother of missing Gus Lamont has revealed the little boy has vanished from his family's remote outback sheep station once before.

Gus, 4, disappeared from Oak Park Station near Yunta, South Australia, on the evening of September 27.

At first, it was treated as a missing child case, but SA Police in January declared it a major crime and stated they believed one of Gus's grandparents was a suspect. 

They did not specify if it was Josie or Shannon Murray. Nobody has been arrested or charged.

Gus's parents, Jessica Murray and Josh Lamont, are not suspects and have pleaded for more information on their missing son. 

Josie, 75, is set to give a bombshell interview to 7NEWS Spotlight on Sunday night.

The network has dropped a preview of the exclusive interview hours before the segment will air, in which Josie claimed Gus had wandered off before - and that she was the one who had recovered him.

'Shan had taken him down to the shearer’s quarters while Jess and I were out mustering and he had wandered off,' Josie said.

'She came home and told us Gus had gone for a walk somewhere... I was the one who found him.'

more to come