In the late 60s I was put, at no notice, into a post that required PV clearance because I necessarily had access to the war plans for the unit. About 4 months later, a plod turned up to do the PV interview, which mostly consisted of him telling me how brilliant the system was and how much better they were at catching people than in the Philby era.
At the end of the chat, he asked when I expected to be in the post for which the clearance was needed. When I said "Four months ago", he blanched. Two weeks later, a story broke in the press of a junior airman at Digby who had been giving away (or selling - can't recall) highly classified material.