So was the home that blew up. It was on city NG service. Many things including barometric pressure determine how fast it rises, if it rises, or if it concentrates in lower areas.
I don’t get your point, we all know that natural gas can explode if confined and ignited.
But natural gas does not puddle, it wants to dissipate, you keep talking about it as though it is propane, natural gas will seek an escape, propane wants to stay in place if it can.