If the outside air temp is around 40 degrees, it is possible that the engine is running simply to keep the battery pack and electronics at the optimal temperature. I know my '13 volt will run the ICE at temps of 37 or 38 degrees while driving down the highway.
On a side note, I'm having a different issue with my '13 Volt, and don't know nearly enough about the car to troubleshoot. I DO know that after fully charging the car on 240, and plug in my OBD-II gauge before I leave for work, it shows the battery at 86-87%.
When I drive the car, using all the EV before the ICE kicks on, I have driven roughly 35-40 miles on EV, consumed ~11 kWh, yet my gauge shows the battery at 18-22%. Until this past September or October, I was easily able to drive 45-60 miles on EV, and the car never dropped below 33 miles of estimated range in the winter. Now, I can never get the estimated range above 35, and My question is why? I have another thread that I was trying to get help on, without much help, so any help there would be awesome ... but for now, I return you to this thread ... sorry for the near hi-jacking.