On 25 October 2021, a Boeing 737-800 had just reached its cruise altitude after takeoff from Perth when a fuel imbalance message was displayed on the system panel. Despite specified indications for a fuel leak as the cause of this message not being met, it was determined that the left engine should be shut down. A ‘PAN’ was declared and a diversion to Kalgoorlie completed. Inspection there found the fuel imbalance was within normal limits and that crew diagnosis of a fuel leak had been flawed. Non-standard closure of the crossfeed valve was suspected as the origin of the imbalance.