It's starting to remind me of the typical carb ice accident where the evidence melts before the investigators get to the carburetor.
Similarly any wax in the fuel and sensor lines likely had time to go back into solution and any ice time to melt
If the sensor lines retained the fuel, the investigators could check it for freezing point and moisture content.
The investigators could also put a temperature sensor/recorder on the lines and see if the sensor lines are exposed to a lower temperature than the tanks. However the proximity to the engine suggests it would be warmer there.