South Australian police have expanded their search for missing four-year-old Gus Lamont to a new family property several kilometres from where he was last seen near Yunta almost five months ago. A cadaver dog from New South Wales has been deployed to search a homestead, shearing shed and nearby national park as part of what police have declared a major crime investigation. The search follows the arrest of Gus's grandparent, Josie Murray, on firearms offences unrelated to the boy's disappearance.