If the engine’s off because of stop/start the warning noise should already be much louder and accompanied by dashboard warnings. See also what happens when you

If the engine’s off because of stop/start the warning noise should already be much louder and accompanied by dashboard warnings. See also what happens when you leave the lights on. That’s what doesn’t make a lot of sense here before we dash off to design some sort of countermeasure - there’s either an engine running (and “modern” engines aren’t that quiet, especially in a confined space), or there should be some loud visual/auditory warning. I guess it’s possible the victims didn’t hear either?