Now that my 2007 Prius has 140K miles on it, I'm monitoring its behavior more carefully then previously and have noticed a couple of odd things on the Energy Monitor. Both things are when the car is at a complete stop, say at a traffic light. First example, the ICE is off and the display shows that power is flowing from the battery to the wheels. This happens for the entire duration of the stop which can be more than a minute. In the second situation, the ICE is running (probably to charge the traction battery) but the Energy Monitor does not show any power flowing. Is this worth investigating further? If so how? I have an OBD II reader and Torque Pro if that helps. Thanks.
For the 1st part, you're probably not pressing on the brake hard enough. For #2, try shutting off the HVAC completely (no auto and no fan). See Why mileage gets worse in winter | PriusChat re: better heater management.
For #2, also check that the 12V battery voltage is OK. Should be initially 14.2V or thereabouts, dropping to 13.8 or a bit less. If it is hanging around 13 or lower, then there is a problem with the battery that is keeping it from getting fully charged (shorted cell). That will keep the engine on too.
I've noticed that my 08 does this sometimes too. I hadn't thought that it was an indication of a problem though. I have been working through a 12V aux battery capacity problem lately with the Optima folks. Still testing and evaluating. When it is load tested (at AutoZone) it checks out fine, but the MFD test will show voltages about .2 to .4 below the minimum recommended. I haven't calculated what the voltage offset is yet, comparing MFD measurements to DVOM readings directly from the battery. I have read that the MFD is typically about .2 volts low. I wouldn't expect a marginal 12V aux to cause problems like this while the vehicle is "ready". I would expect somewhat lower mpg with the ICE trying to charge that battery. I wonder what a Toyota Service Advisor would say about this strange Energy Monitor display. They would probably want you to duplicate the indications or record a video of it misbehaving.
Both situations are absolutely normal. The first one will go away if you press the brake pedal harder. The second one is because the ICE needs to heat up either for emissions purposes or to provide heat for occupant comfort. It's not charging the battery because the battery is likely already in the "sweet spot" of 6 bars.
