i have left the house for 20 mins, come back to find my oscar with half its scales missing, bleeding from the side, i think something in the tank fell on him, but i'll never know.
i also have lost 5-6 fish over the last ten years to spontaneous deaths all with same symptoms, dizzy, lose colour, fins and gils clenched, and death within 4-5 hours of symptoms appearing.
mystery deaths are an inevitable part of fishkeeping. best not to get too caught up on it. never get attached to a fish!