There is gas, gas, gas, gas and gas and they all smell different
Sewer gas (Black/gray tanks) is mostly methane smells like, well I'll let you figure that one out.
Batteries give of H2S (Hydrogen suflide) Essence of rotten eggs
Water heaters can do this too (The hot water)
Propane.. I'm told the odor is "Essence of skunk" but I've smelled it. and I'd not describe it that way. Alas I know it when I sniff it. but... I can't describe it.
Turn on Stove burner for like 1-2 Seconds (no longer) without lighting (propane on of course) and sniff... THen let it air out a bit. NO SMOKING or sparks or flames while doing this (though 1-2 seconds you will likely be safe.
Remember the answer I gave to the question
What happens if you mix air and propane in the proper ratio and there is a spark or flame?
Answer: You land 2 counties over
(room breakes down in laughter)
Instructor "Exactly"