A high-level investigation was launched amid fears that "a mole" was leaking sensitive Government papers to Peter Robinson in the early 1980s.
The probe came after the DUP MP appeared to have been briefed on the contents of a secret report relating to housing demand in his East Belfast constituency.
It was the second time in six months that official documentation was apparently leaked to Mr Robinson and the incident prompted a flurry of anxious letters between officials at the Department of the Environment (DoE), and led to a major behind-the-scenes investigation.
An RUC investigation into whether the now First Minister had breached the Official Secrets Act was ruled out "given the present political climate".
Within a fortnight of the incident on January 15, 1981, an inquiry had been set up into the alleged leakage.However, it was unable to determine the source of the leak.
Nowadays, as First Minister, Mr Robinson takes a severe stance on leaks.
Last month he expressed his displeasure at "continued leaking of classified documents into the public domain and particularly to representatives of the media".