The smugglers, if they stop for a break, generally do so in Lak Sao, the last city in Laos before the Vietnamese border. "You hear the trucks before you see them," the locals joke, as the convoys stream along these dusty roads almost every night, the howls of their cargo echoing through the cool mountain air. Following the route by public bus, I spot a lone dog truck travelling with empty cages on its way back to the Thai border.