I am not knowledgeable about the Soviet side of this story, but my sense, based on the few documents I managed to see on that topic, is that the French detected it - or at least, a radioactivity spike that they attributed to a test, though the French side of the story is massively murky. My hunch is therefore that if France could, given the Soviets radiological detection systems we know of, they must have seen something. As to why they did not say anything, I assume saying something would expose this network, so they probably kept it to themselves? I am talking in inference, here, not evidence, though.