There are no poisons that are completely untraceable. That is a common myth that many people believe. There are, however, some that are more difficult to detect than others. A list of those would include arsenic, selenium, and plants such as deadly nightshade and oleander. This is because toxicology tests are not normally sensitive enough to detect those types of poisons. If, however, foul play is suspected then further testing will be done and those poisons will be detected.