I think that maybe you were not paying enough attention, im sure they do pass wind , it probably did it while u were asleep, (I take it that you dont have a round the clock vigil just in case there is a a windy moment ). Maybe its an issue that no one has really thought of before. i have never heard the sound of my dinner dropping in my belly but im sure it happens , my advice would be not to worry about it , theres far more exciting stuff going on around...