A family heading for a holiday in the south of France drove 150km (93 miles) before finding out they had left their three-year-old daughter behind at a motorway picnic area.
Even then, they only realised because of an alert broadcast on the radio.
The girl was abandoned on Sunday afternoon at the Bras de Zil rest-stop south of Valence.
By the time another family had found her, the girl’s parents were already well on their way to their destination.
The girl was apparently left at around midday (10:00 GMT) near Loriol, at a picnic stop beside the A7 motorway known as the “autoroute du soleil” (motorway of the sun).
The family who found her waited some time for her parents to return, but then alerted local police.
The girl told officers she was on her way to the beach, had a brother and sister and had seen her dad’s car drive off.
It was not until police broadcast an appeal on a motorway radio channel that her father contacted them at around three o’clock. The family had already driven past Aix-en-Provence on their way south.
The family was eventually reunited more than two hours later.
Local prosecutors were considering whether to take action against the parents.