There are no completely untraceable poisons. There are, however, some chemicals that are more difficult to detect than others, including arsenic, selenium and plants such as deadly nightshade and oleander.
Toxicology tests are not normally sensitive enough to detect the types of poisons listed above. If, however, foul play is suspected in a criminal investigation, then further testing is conducted, and the poisons are most likely to be detected.
Its discreteness and potent nature give arsenic the title of "king of poisons." As it is extremely difficult to detect, it is often used as murder weapon and is featured as a common trope in literary fiction.