If you have Nav on the Highlander it works and looks just like the Prius. If you have 4WD you have the rear motor added along with its power flow. Without Nav, you still have a subset of the Nav graphics under the speedometer. I don't use the speedometer graphic since the Nav screen includes that information. The speedometer graphic includes all the power flow except the ICE charging the battery. If you achieve no power flow between components, you will be able to see that on the speedometer graphic.

You're right, the only time I could imagine that being the case was when the battery was starting the ICE but that really isn't a condition the display would indicate.