You sure the Big Six weren't aware of talks with the Russians? I don't see why they wouldn't be, given that they had agreed to cibtact the Soviet Union a week earlier at a meeting (on June 22).
June 24, Vice Chief of the IJA (direct subordinate to the chief of the IJA, a Big Six member), written message to the Japanese foreign ministry: "As we have said before, Japan is firmly determined to prosecute the Greater East Asian War to the very end. There is a report, however, to the effect that some Japanese official stationed in Sweden is making peace overtures to America. That is demagoguery pure and simple, and if you have any idea as to the source of those reports please inform us."
Officially, the Big Six weren't told that Togo was talking to the Soviet Union until August 9 meeting. That morning they were discussing the ramifications of the previous day's declaration of war by the USSR, when a messenger interrupted them to state that the Nagasaki bombing had just occurred. Either they broke temporarily and met later, or at that exact meeting, Togo admitted to the rest that he'd been in talks with Soviets trying to get them to get a line to the US, and that he had done it at the orders of Hirohito.
Note. These WERE NOT SURRENDER GESTURES. Even after the Nagasaki bombing, the Big Six could not even decide what to do for days, and for much of that time the pro-war ministers of the Big Six were still pressing for terms that were insane. Self disarmament, no occupation, Japanese control of war crime trials. After everything that had occurred to Aug 9-10, three of the seven most powerful men in Japan, whose votes are needed to make any decision, still believing they could end the war with those terms is blatant indication they were not trying to end the war.