I did read a long time back about people using pizo elements as microphones to pick up sounds well above human hearing and then lowering the pitch uniformly into the audible range. One of the things they reported as sounding interesting was gas flowing in a pipe. So there is some basis for building a sensor around this idea.
I would want to have a discussion with my local gas supplier before I stuck sensors to the glass of my gas meter! Could be easily misunderstood by the meter reader!