Really? Must have missed that one ... in that case I withdraw the question, someone else can go!
In case anyone is interested, I'll give the answer - Victor Suvorov told that tale in Icebreaker, I'm not sure I believe it, but maybe it's true...
According to him, Stalin's spymaster had agents checking the price for sheepskin and mutton in western markets daily. He reasoned that, if the German armies massed on the border were to actually attack the USSR, they would need to be equipped with winter gear, which would necessitate large numbers of sheepskins. If the price of sheepskin went up, while the price of mutton sank at the same time (due to mass slaughter of sheep), that would be an indicator of an imminent attack.
Since Hitler and the General Staff believed in a quick victory and didn't buy winter gear, no such market fluctuation appeared, and so he assured Stalin, right up until the actual German attack, that no preparations had been made and so no attack would occur.