We all know how repulsive our noses find the material (Gas, Liquid AND Solid) that is regularly (one would hope) expelled from our bellies via our poopchute. Me thinks a certain smuggler was relying on that fact when he or she (hey, we may not be conventional on this site, but we're certainly P.C.) secreted over 200 pounds of cocaine beneath the ship's "sanitary holding tank".
Maybe he or she banked on the fact that even IF drug dogs "hit" on that location, the handlers would "ass"ume it's because of the dogs' proclivity toward the hindquarters of ALL species.
Maybe he or she trusted the human inspectors ingrained fear of feces (we all have it), thereby offering a biological (if a bit Freudian) mask to their game.