Thx, I'm no good at that stuff, but would I be right in thinking that if it was mains gas, it would be natural gas, and rise, whereas if, perhaps for heating only, the bakery had some propane in tanks, then that would have sunk? I'd have assumed that if natural gas was available from the mains, the bakery would prefer that, with electric being very much a second best.  However for background heating, since a bakery might be hot enough from the ovens, a secondary propane system might be used ? If both those are right, then a propane leak would pool somewhere like a basement, and what we saw, might be a result? Still just guessing of course...