husbandry/labs
There's several gasses that could do it. Carbon Dioxide, argon. Most of them are used in lab sort of situations where they purposely put the animal down, but I think unless there was foul play, we can assume that isn't the case here. Most commonly in pets, it would be something lodged in the throat, but we concluded that wasn't the case.
All of our evidence keeps giving us more questions than answers, I'm afraid.