A postal worker -- who is stationed at the regional sort facility for East Texas told me they sometimes use dogs to check bundles of packages before they are processed. He said, "They can detect substances that are used in explosive devices or in this case the odor that sulphur gives off inside a closed container." I am only taking his word for it and he didn't elaborate what kind of dogs are used to do this public safety duty. I guess that's why every time I have a large envelope going first class, the clerk always asks is there anything hazardous or potentially dangerous included in the package. Every clerk asks every time. Sort of a standard question asked of ever postal patron mailing a large envelope. Try this experiment: Enclose several packs of matches in a plastic sandwich bag, and let stand a couple of days. When you open and take whiff of bag, the odor smells like rotten eggs -- that's sulphur gas. If you have ever been to Sulphur, LA, your nose will give you a real sense of what sulphur smells like.