Holding power-volume-up for a second to reach the “power off?” screen is enough to kill Touch / Face ID instantly, at which point you can let go without having

Holding power-volume-up for a second to reach the “power off?” screen is enough to kill Touch / Face ID instantly, at which point you can let go without having fired an alarm (unless you’ve gone and altered your alert settings, so test this when a loud noise won’t endanger you).