Hi Ozwards
The owner's handbook is a bit ambiguous, but my interpretation is that a Left or Right cab door will give one message and a Side or Rear (= load compartment) door will give a different message.
The side door seems to more often be the culprit. There is a plunger switch plus 4 electrical contacts mounted in the bodywork at the trailing edge. Sometimes this responds to being packed out behind with thin plastic, as the metal it's mounted on is thin and prone to bending. From memory it's held with plastic rivets so some proper screws with shallow heads may also help. The metal contact faces can also be cleaned up.
There is another switch within the lock at the rear, but I suspect this wouldn't be activated unless the door was fairly obviously not shut.