Not necessarily a drug interdiction question, but a question I often ask is :
are there any dead bodies, bombs or bazookas in the vehicle? I have found that if someone is trying to hide something they often don't realise you are asking such "unusual" questions and will be serious in their replys. People that often have nothing to hide, will laugh when you ask the questions. As with everything, it doesn't always work, but I have found it useful.