That's when all the hot antifreeze leaked out and evaporated into the garage and into the automobile's interior.
It was everywhere.
Then, she came downstairs in the morning to go to the airport. Hopped in the car, put her key in the ignition (or maybe this had one of those chip controlled ignitions that starts up when your "key thing" gets in proximity. So it starts up. She puts it in reverse. As soon as that heating element in the catalytic device kicked up to 711 degrees (what, 20 seconds tops?) everything blew up. She would have been both burned and suffocated simultaneously, and probably knocked unconscious.
If it was that easy, wouldn’t it be happening a lot?