[ARCHIVE ANSWER]
Cannabis molecules will permeate through different containers at different rates, depending on how porous the container is. You can take cannabis and shrink wrap the hell out of it, stuff it in industrial PVC pipe and cement caps on then stick it in the fuel tank of a vehicle fully submerged with gasoline, diesel, or whatever fuel you use, and the cannabis odor will eventually permeate and be detectable to a dog. A dog can differentiate smells just like a human can visually differentiate objects that are grouped together.