I don't know the answer to your question, but I suspect if you do end up too decomposed for a cause of death to be found, authorities would probe your death in other ways, as they'd be very keen in that instance to eliminate foul play. Your electronic devices, bank accounts, and other online activities would probably be looked at. They might find out things you'd bought online, which would give them clues. Deleted internet history is often recoverable, and if it isn't, history of your bank account will most likely provide similar information anyway.
You're also going to have the fentanyl/SN on you at the time of death, so I think it would be difficult to prevent the possibility of there being containers/packaging nearby with, at the very least traces of it on. People talk of feeling drowsy and having to lie/sit down just minutes after consumption, so I would think it would be hard to get rid of the evidence in time.
Sorry if it's not the answer you're looking for, but I think with SN/Fentanyl you'd have a hard time concealing your cause of death. Not necessarily impossible, but definitely hard.