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?