Answer: The stranger was a baby and the woman died in childbirth.
A man and a woman were driving in their car when it broke down. The man decided to go for help at a gas station a few miles back. He made sure nobody was in the car, rolled all the windows up, and locked all of the sedan's doors. He went off, but when he came back, his wife was dead, and there was a stranger in the car. No physical damage was done to the car, so how did the stranger get in?