Truly undetectable poisons are stuff of fiction given modern chemical and forensic knowledge. There are some toxins that are extremely difficult to detect because their LDs are so small (ricin, for example, can kill in incredibly small amounts and its effects rapidly become untreatable, but it takes 32-76 hours to actual die following exposure and it's far from a peaceful or painless death) but they're still traceable and you can't just buy them since they're considered biological weapons or are otherwise strictly controlled. Insulin poisoning, air embolism, and any other methods at mimicking an accidental death that require injection will leave a mark. An opioid overdose could be passed off unintentional if you have a history of usage, I guess?
I would strongly advise against food poisoning as a method. Even if treatment isn't sought, it would be days of painful symptoms before death finally occurs from dehydration or organ failure.