Benzos are easily detected in the blood, for one. There are blood tests for just about everything. Your methoglobenemia level will be off the charts, something very unique to very few substances. For another, part of an autopsy for unexplained deaths is checking stomach contents. They will find everything in there as you will die before your body has emptied your stomach. Nobody is going to suspect that a young person just randomly died in their sleep, there will be an autopsy and they will piece it all together.