Engine Exhaust System problems of the 2009 Buick Lucerne

The Buick ws in my garage which is attached to the house, before going to bed I always open the door to the garage to be sure the garage door is down and the flag is inside. Did so that night and all was normal. The next morning when I opened the door to the garage to put the big door up and the flag out the garage was very warm. No other indication of a problem, assumed he furnace had acted up. We took the car to have breakfast did notice the hood was very warm as if the car had been running (I do not have a sense of smell so did not smell an exhaust) fuel was very low ( I wondered if there was enough to get to a service station) filled he car had breakfast then came home not apparent problems. Later that morning my wife use the car to go to an appointment when it started to miss, she pulled off the road and called aaa who towed the car to the nearest Buick dealer. The car is there now, they had no yet determined the problem.