Perhaps a geyser had broken through the ground and the bottom of the pool. Or any kind of volcanic vent that could heat the earth under the pool above the boiling point of water.
My first thought: a gremlin.
Tl;dr: No, you should only drain it so that the water level is ‘below the skimmer level and jets to prevent plumbing freezes.’
It’s gone now; the pool is calm.
Outdoors.
The Lloigor has collected all of the psychic energy it can consume and escaped.
Stranger
Well, who knows if it was a noxious or poisonous gas unless somebody comes and checks?
Obviously management didn’t care.
If they knew what it was, putting the info out so apartment dwellers would not do something silly, like, say, call authorities.
Their bad.