"apparently departed William creek at 5 pm."
The logical thing for this guy to have done is simply this: Refuel his aircraft at the self serve YCBP bowser and then land at YWMC for the night. (Trevor has some great new self contained, very comfortable single dongas for only $120 per night.)
Next morning, he could have had a picturesque flight over Lake Eyre.
I've visited William Creek on only six occasions. (The latest at the end of January, this year.) But the place is rather attractive due to the lack of mobile phone access...
A 406 MHz PLB is very cheap to buy and may save your life. Don't forget to register it with AMSA.