EV mode above 42 mph

Just taking a shot in the dark here....

It could be that the Prius keeps the fuel injection system pressurized even when the ICE is off. If that's the case, then maybe fuel is slowly making the rounds from the tank, through the fuel system, then back to the tank through the return lines, which every car has these days. If that's the case, and depending on where the sensor is that the scan gauge is picking up, I could see it registering a very small flow rate.

But that's just a total guess...I have no info to confirm or deny this idea.

EDIT: I could definitely see this being the case if there are two sensors in the car...one somewhere in the line from the tank to the engine, and one in the return line. If the car measures both and does some internal calculation to determine how much fuel is being used, perhaps the scan gauge is getting the raw result of that calculation, without benefit of some further calculation the car is doing to determine the fuel economy numbers we see displayed on our dashboards.

 

Typically - not sure about the Prius - fuel flow is calculated by looking at fuel pressure (if it's even sensed - if not, it's basically going to be assumed as what it's regulated at, so a weak fuel pump will be seen as increasing fuel consumption by the computer if the pressure isn't sensed, longer durations being required to maintain stoichiometric ratio) versus fuel injector duration.