Confidential documents have been stolen from the boot of a car belonging to a Kent Police officer.
A member of the public found the paperwork dumped in the street the day after the theft and took it to a local police station.
An investigation by the Information Commissioner concluded the force needed to improve its safeguards for documents taken out of the office by staff.
Kent Police said the officer had been disciplined.
In a statement, Assistant Chief Constable Allyn Thomas said: "An officer made a mistake by leaving confidential documents in a locked briefcase that was stolen from the secure boot of his car.
"He has been the subject of disciplinary action and has received further training regarding data protection.
"As a result of this incident we have also re-examined our policies and procedures to ensure this does not happen again."
The force has refused to say where the incident happened, or when, and the rank of the officer involved.
The documents were stolen from the car while it was parked overnight at a residential address.
An investigation by the Information Commissioner's Office (ICO) found that the officer had not used his secure briefcase to transport the papers, nor had he been provided with a secure storage facility at his home.
It said Adrian Leppard, temporary Chief Constable of Kent Police, had now signed a formal undertaking to ensure that staff whose roles require them to have access to confidential information outside the office are provided with secure transportation and storage facilities.
Sally-anne Poole, enforcement group manager at the ICO, said: "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."
A lack of awareness of data protection requirements can lead to personal information falling into the wrong hands, she said.