They only twigged she was missing when an alert was issued on French radio, officers said.
The child was found by other holidaymakers in a rest area off the A7 "Autoroute du Soleil" (Highway of the Sun) near Loriol in the Drome region of southern France just after lunch on Sunday.
All she could tell them was that she had a brother and a sister and that she was "going to the seaside" when she saw "daddy's car pull away", a
gendarme told AFP.
It was only when they heard the alert -- 45 minutes after the child had been found -- that the parents called the police and turned around to drive back up the motorway.
"The family had passed Aix-en-Provence (150 kilometres or 93 miles away) and were going towards Saint-Raphael and none of them had noticed she wasn't there," the officer said.
The parents were being questioned Sunday evening by officers at the police station where the child was being looked after.
"We are going to listen to what they have to say and talk to the prosecutor at Valence to see if this should be taken further," the officer added.
Millions of French families take to the roads during August's annual holiday rush to the sea and mountains, putting often unbearable strain on the country's motorway system and their own nerves.