Riddle 23: The Stranger

A husband and wife are traveling on a deserted road when they run out of gas. The husband leaves the wife in the car to go get gas. He makes sure the doors are locked and the windows are closed before he leaves. But when he returns, there is a stranger in the car with his wife. His wife did not unlock the doors or open the windows while he was gone. Who is the stranger and how did he get there?

NOTE: The correct answer has been guessed. To read it, see the comments section.

Lateral Thinking Puzzlers by Paul Sloane has lots of classic riddles, clues and answers. Click here to order it.