At autopsy, the existence of any drugs (and alcohol) in the body will be discovered. If the drug is uncommon or difficult to obtain, questions will be asked about whether or not assistance was provided in obtaining, preparing or administering the substance.
Families rarely have control over whether an autopsy is performed. The laws governing autopsies generally have little regard for the feelings of those left behind. That said, autopsies are expensive and so are generally only carried out if there is a legal or medical mystery associated with the death.
In cases where the death is obviously a suicide, an autopsy is generally unlikely to be performed. The take-home message in regard to autopsies is that if a totally undetectable death is important to you, and your family is prepared to stretch the law for you by taking the equipment away after your death, Nitrogen is your only option that will show nothing at autopsy.