''We could only prove that the bank had a vague sense of crisis,'' said one investigator. Last week the president of the Tokyo Stock Exchange, Michio Takeuchi, said similarly that no charges were likely in the case of Sankyo Seiki, the world's largest maker of music boxes and now a major electronic components manufacturer. Two weeks ago, the stock price soared 10 percent just before Nippon Steel became Sankyo's biggest investor, acquiring an 18 percent stake in the company. Among market experts here, no one is surprised that early word of the Sankyo-Nippon Steel deal leaked out. In fact, one American trader here noted, ''The system assures that it will leak out.'' The Japanese system points up how the American and Japanese markets are alike only in appearance. In the United States, investment bankers and floor traders are supposed to be strictly segregated, so that a deal-maker cannot profit from insider knowledge.