I think I'd agree with you, if the engine is cold and the car wasn't run for 12+ hours, there shouldn't be much surface charge and your voltage measurement should be accurate. It's not likely the voltage would rise after leaving the headlights on a while. Is the 12.2V you read from the cigarette lighter readout? I would double check with a DVM at the terminals just in case, big difference in charge state with just 2-3 tenths of a volt. Based on the dealer's measurements, 12.32V, 62F, I estimate the battery was at around 65%, a little less than the dealer's 75%. Like the others have stated, I would also suggest topping off the battery overnight and periodically check the resting voltage. If it keeps dropping, might be some parasitic drain (or ultimately just a dying battery!) and you'll get a better idea of whether you want to keep running the battery.