Back in the Cold War times, flying a BAC 1-11 from West Germany to West Berlin Tegel the USAF ATC called up, “Speedbird 1234 pop up traffic 12oclock 2 miles”. At night the Soviets flew uncontrolled through the air corridors linking Berlin to civilisation. My response, “Roger, thanks”, meant we hadn’t hit, though I recall the slipstream buffet as something unseen in the dark crossed our path.