The Energy Monitor does not tell you all that the engine is doing.
1. Engine is running to propel the car - on Energy Monitor.
2. Engine is running to recharge the battery - on the Energy Monitor. (BTW: Classic Prius usually hangs out at the 75% charge level all the time, occasionally the 50% point, and only the full or 1/4 in large hilly areas (or 1/4 if stuck in traffic idling long enough).
3. Engine is running to warm itself up for emissions reasons. If the blue cold coolant light is on, your engine is probably running. About the first 5 minutes of a trip, maybe up to 20min if it is really cold outside and the car has been sitting outside overnight. will not show up on Energy Monitor.
4. Engine is running to provide "waste" heat for the passenger compartment - will not show up on Energy Monitor
5. Engine is running to help calibrate charge levels in the hybrid battery. (This is seen in Classic Prius when the engine suddenly starts up when you put the car into Park...) It may or may not show up on the Energy Monitor...
6. Engine is running to power the AC compressor via belt on the Classic Prius. If you have the left fan position dial on the windshield for front windshield defrosting, that will use the AC for dehumidifying. Windshild/floor is less agressive in the AC compressor use. You can also get the engine to run almost constantly if you use the MAX button for the AC, whereas the engine/compressor will just cycle as needed if you only used the regular AC button. Running the AC compressor will not show up on the Energy Monitor.
So, how cold was it this morning? How long was your drive before you noticed the engine on at stoplights? What were your climate control settings?