No suspicious circumstances but not surprising in this area during the summer. There's absolutely no surface water, very little chance of finding food and navigation is extremely difficult. You simply can't tell people how difficult it is to find your way in this terrain because most people trust their basic instincts. Even if you are lucky enough to find a bit of shade for your camp that only means that your camp is better hidden.
I don't know the exact spot where they went missing but we were in the area just after Christmas with our bus, three metres high, attached to the dual quad trailer. I nosed the bus under the biggest tree on the edge of a bottle dump and we walked about looking for relics for the Cue Heritage Centre. On more than one occasion we lost track of where the bus was and had to walk the perimeter of the bottle dump to locate the bus.
This is open country with very little vegetation, almost no hills or gullies and we never went further than 150m from the vehicle. The only thing that brought us back to camp was the limited size of the pile of cans, glass and relics. As long as we stayed within the dump we were near the vehicle but there was just enough mulga to confuse us. I carried a PLB but Mrs M still thinks she can find her way back even though the evidence proves otherwise.