The car could have been rummaged through to appear like a burglary gone wrong. The removal of items would give that impression but as far as locking it goes I'm not sure of the significance. If it was someone known to the victims perhaps they were counting on the car coming back into family possession and why burn it when it can be sold for a sum?
Do we know if the truck was fingerprinted before returning it to his family or was it only fingerprinted when it was retrieved from them? it looked pretty cleaned-up when it was returned to them the first time.
Would the police drive the truck back or ask a family member to collect it from Wonnongatta?