Bus Etiquette question

What, exactly, is the cor­rect response to take when you’re on the bus with a phantom farter?

I was on a very full #4 on the way home the oth­er even­ing, and for most of the trip, at reg­u­lar inter­vals, the over­whelm­ing pong of a bot­tom-burp would waft its way through the throng.

Of course, if one is of a guilty dis­pos­i­tion like myself, you assume that every­one else assumes that you were the guilty party.

How does one com­port one­self as to make it pat­ently obvi­ous that the cul­prit is not, in fact, one’s own bot­tom?