OK. You've got a leak

It's also possible, I suppose, that if the pipework goes to the ground floor it might actually go into the foundations, and be leaking into the ground under the building. Which again could absorb a lot.

Porous structures like concrete and earth can hold a large amount of gases such as propane or butane, which if they are sufficiently concentrated to catch fire, can burn for a remarkably long time and be quite difficult to extinguish. Don't ask how I know...

But in my defense, I was 12.

pca