Smell occurs on the molecular level. There is NO way to hide it. Hence, a dog trained on any particular scent can detect that substance be it covered in plastic, then encased in wax, then suspended in diesel fuel.
How near the dog has to be, who knows, 10 feet?
Bottom line, if a dog is involved, you're f@cked!
not quite right... this really depends on how long the package has been sealed. Dogs can't smell through plastic, but eventually anything odorous is going to get through the plastic, at which point the dog can detect it.