Passing Gas In Prayer
The criteria is the same that we have been discussing all along i.e. one needs to be certain about happening of that event. In this particular scenario, sound and smell are two things which make it certain. But as you said “there can be cases where a person is absolutely sure that they’ve passed gas, even without smell or sound“, therefore, if a person is certain about it, then it should be considered as such even without sound or smell.