The documents were found in a room used to store musical instruments
An inquiry is under way at an open prison after an inmate was able to remove documents from a disused office.
The documents, thought to be over five years old, were taken from a room used to store musical instruments at Ford Open Prison, West Sussex.
They were found after a prisoner involved in an internal security matter had his cell searched.
The Ministry of Justice (MoJ) said an investigation was under way and a full search of the prison was conducted.
Denis Lindsell, of Ford's Independent Monitoring Board, said the prison-wide search was launched after the documents were found.
"A search took place which involved approximately 15 prison officers from other prisons because it was understood that some documents had been taken from a disused office," he said.
"The particular office had not been used for some time and, obviously, a check had not been made on it properly."
It is believed that a quantity of contraband alcohol was found during the search of the prison.
The MoJ said in a statement: "We take the protection of data very seriously.
"An investigation is under way and a full search of the prison has been conducted."