The Prime Minister, his wife and three children had enjoyed an afternoon meal with two other families at The Plough in Cadsden, Bucks, when they took eye off their eldest daughter Nancy.
When the family gathered their things and left Mr Cameron shared a car with his bodyguards while Samantha followed behind with son Elwen, six, and daughter Florence, 22 months.
The Prime Minister thought Nancy was in the car with his wife, while Samantha thought she had jumped in with her father. It was only when both cars pulled up to Chequers, two miles away, that they realised she was not there.
A Downing Street source said the “distraught” parents rushed inside and telephoned the pub, where staff reassured them that their daughter had been found alone in the lavatory and was being looked after.