Documents containing information on violent and registered sex offenders were inadvertently left on a bus in Brighton by a Prison Service employee.
Sussex Police were informed that the file, containing the cases of several people being managed by the Multi-Agency Public Protection Arrangements (Mappa) authority, were reported missing by the employee who had left them on a bus in central Brighton.
Police identified the vehicle concerned and after checking CCTV found that the documents had been picked up by a 32-year-old man who was arrested in Brighton on suspicion of theft.
A police spokesman said: "He explained that he had handed the documents to The Argus newspaper shortly after finding them on Tuesday afternoon.
"Police contacted the newspaper at around 10am and made immediate arrangements to retrieve the documents, which they quickly returned after being asked."
The arrested man was released later and no action is being taken against him, police said.
Sussex Police and the Surrey and Sussex Probation Trust are taking steps to ensure the safety of people referred to in the documents, and to reassure them that they have been recovered, the spokesman said.
The spokesman said: "Sussex Police is investigating how the documents came to be mislaid and will be sharing these findings with the Prison Service and other agencies as part of a review by Msppa partners."
Nick Smart, Surrey and Sussex Probation Trust chief executive, and co-chairman of the Mappa strategic board, said: "The Multi-Agency Public Protection Arrangements are specifically to manage any risks posed by offenders, so we take our responsibilities to keep people safe and the security of the information we hold very seriously.
"Mappa partners have already met to start a review of existing procedures to see whether there are any changes we could make to strengthen security even further."