In Albania, he heard rumours of a massacre in a Kosovo village called Little Krusha - 100 men shot down in a hay barn - and went in search of proof. He found Granny Batusha and what was left of her family: her sons' wives, her grandchildren. All the adult males, from teenage boys to old men, had been taken. Granny Batusha could provide evidence that these men had once existed, in the shape of a wedding video which showed them drinking and dancing; but nobody knew where they had gone or what had happened to them.
From there, Sweeney went in search of the only person who could prove that the massacre had taken place: a man who was said to have survived the slaughter. Sweeney found him, and heard his story - how the men of the village had been taken into Granny Batusha's barn and machine-guns had opened up on them. His story included some horrible details: a man whose arm had been severed by bullets trying to push himself up from burning straw, his hands on fire as he jumped out of a window. An old man offered more examples of pointless butchery: hiding in the hills above the village, he had seen a Serb take a scythe to a cow, chopping off one of its legs.
The edge of horror was partly dulled by Sweeney's exaggeratedly downbeat delivery, too conscious of his own dramatic effect. But he made a necessary point: proving anything about Kosovo is going to be difficult. Sweeney had a hard enough time tracking down his witnesses; what happens now if the Serbs claim, as they have many times in past weeks, that these men were saying what they were told to say, what it was in their interest to say?
We have stories, a wedding video, children crying for their fathers: enough to make it all seem a little more real, but not enough to stand it up in any court. Believing only what we see may not always be the sensible course.
The importance we place on what we can see was emphasised in The Planets (BBC2), which this week went to the Moon, thankfully avoiding any mention of small steps and giant leaps along the way. Seeing for ourselves was vital to our understanding of the Moon - until we sent cameras, and then men, there the green cheese theory was still in the running. But it's interesting, too, how what we can see has shaped this series: now that the Cold War is over, and we have access to Soviet film archives and Soviet scientists, the significance of the Soviet contribution to space exploration is being given some overdue recognition.
Indeed, the star of the series is Sergei Khrushchev, Nikita's boy, a jovial rocket scientist now based at a college in Rhode Island. You wonder if he shares his father's politics. This would, of course, make him a Rhode Island red.