Meanwhile, when the boy's mother, Leanna Harris, arrived at a day care center to pick the boy up, employees there told her Cooper had never been dropped off, the detective said.
"Ross must have left him in the car," she replied, according to Stoddard. Witnesses said they tried to tell her many other things could have happened, but Leanna Harris insisted that Ross Harris must have left him in the car, Stoddard said.
http://www.cnn.com/2014/07/03/justice/georgia-hot-car-toddler-death/
How did she know?