Natural gas: matches; pilot lights; noses; maybe a (old-style) ringing
telephone;

My house had a very clever electricity shutoff. They put pvc water
supply to the bathroom in this long, round-about path around the
basement ceiling (many changes over the year, an no consolidation --
just keep adding things here and there). The last stretch was about 10
feet, with no supports, pointing right at the main service panel,
ending with a flimsy elbow (again, just hanging there), then going up
through the floor and to the sink (still no supports), mounted on a
small cabinet (not fastened down in any way). Net effect. Give the
cabinet a good kick, break the elbows, squirt the panel, kills the
power, shuts off the well pump, and quickly depreassurizes the water
system. Yay.

So, as long as I am rambling: I hear mixed opinions about smoke
detectors too (though no one official wants to admit as much out loud).
Apparently the beeping won't wake nearly any kid under 5 (though a
parent's whisper usually can), and doesn't wake a lot of adults either.
That, and the ones at work are so loud several of us have actually
considered filing OSHA complaints -- my ears ring for literally hours
after a fire drill!

Don't forget that CO is colorless and odorless. Natural gas is just
colorless, but is made to smell bad. The water and electicity probably
aren't life threatening usually -- not in the way gas would be. I long
for a water shutoff in case of broken pipes while gone on Christmas
vacation.