[lamontcg]

Cyanide salts would probably do it. Last adult who takes it chucks the container into the brush as their last action. The first responders have no way of determining that was cause of death, and it'd wait on the report from the corner (which will take days) or for someone to find that piece of evidence which tests positive for the chemical (which will also take days). There's 1,500 murder-suicides in the United States every year. A naturally occurring bubble of gas that kills an entire family is something that tends to happen zero times in a year. Poisoning as a means of murder-suicide is going to be uncommon but we can all think of Jonestown and Heaven's Gate.

We just don't like to think that is possible, but the coroner has probably seen enough horrible shit go through their table that humans do to each other than they'll have to take the time to rule it out.