Natural gas rises to the top & builds from there. Propane is heavier and builds up from the floor. Interesting that neither one noticed the smell.
Years ago, my neighbor who was a carpenter came over to look at replacing some outside paneling. While on the phone with him, I could hear his wife saying something. Turns out he smelled a gas leak at my house a week earlier when he was checking it out!!
Said it was no big deal since NG rises but when I called the gas company, they told me to evacuate the house, came out within 30 minutes & installed a new gas meter since that’s where the leak came from.
Yes, that is why they ADD the sulfur to the gas. So you can smell it if there is a leak.
I had a gas(propane) fireplace at my former house. I had the regulator replaced twice because I kept smelling gas outside near the tank. The tech said sometimes the regulator is just faulty.
It was leaking too because they came out to fill the tank in the summer when I had turned it off after filling it in the spring. They ended up crediting me for about $50 of gas that had leaked out when the valve on the tank was turned off.