I would say I'm in two minds about the triggered background object, because I envisioned a gas flame that only reaches full power if a lemming walks over it, incinerating them, but at the same time it must act like a fire object and kill all lemmings, the idea being that the triggered object is the flame roaring to full power, while a separate, mostly invisible object, probably with just a small pilot light, is the actual fire object. Being able to disable that background object would ruin the effect a bit (it's fine to be invisible in clear physics mode).