The nature of the information was not disclosed. But with all the data in physical document form, the news may serve as a reminder that paper is still as easily lost as memory sticks and laptops.
The Information Commissioner's Office said the files had not been stored in a secure briefcase. The police force was also blamed for not providing safe storage to the officer at his home.
The documents, which had been in the boot of the car, were found later the next day by a member of public.
Adrian Leppard, temporary chief constable of Kent police, has now signed a formal undertaking to make sure that staff who require access to personal information outside of office hours are provided with sufficient secure storage.
"It is essential that police forces ensure the correct safeguards are in place when storing and transferring personal information, especially when it concerns highly confidential information," said Sally-Anne Poole, enforcement group manager at the ICO.
"A lack of awareness of data protection requirements can lead to personal information falling into the wrong hands."
West Berkshire Council also experienced similar difficulties this month after it lost unsecured USB drives, leaving information on children in the borough at risk of being misused.
Police and government services have been involved in numerous security breaches since January. Eastbourne Borough Council, Glasgow City Council and Kings College London, have all signed formal undertakings with the ICO.