Deaths by unexplained arrhythmias do occur, and one such highly publicized incident was when professional wrestler Jerry "The King" Lawler sustained a heart attack on live television after wrestling a match earlier in the card. He closed his eyes, and the next thing he knew, he was in a hospital bed. He had no blockages, nothing which an autopsy would have been able to explain, his heart simply stopped beating spontaneously. He's speculated that absorbing a succession of elbow drops to the chest during his match may have precipitated his heart attack, but there are clips of him back at the broadcast table right before his heart attack putting on his head set, and he looks completely normal and sounded completely normal on the microphone right before the sudden loud snoring began which signaled the heart attack that caused his heart to stop beating for around 20 minutes. (If he'd been alone, he would be dead.)
Hypothetically, an assistant with a large tank of pure nitrogen and a blower could blow pure nitrogen onto the face of the decedent, then scram with the equipment, leaving no trace of any crime.
On an episode of Chicago PD that my parents were watching, an entire sleeping family was knocked out by a burst of nitrous oxide into their house via the outdoor ventilation system. Artistic license required that the burglars left telltale clues behind, but the story very easily could have been a completely unsolvable murder with the use of industrial nitrogen and no home invasion.