If the fire were to be used in a soundstage and there was smoke, they could easily filter the smoke out. I work for TNA & they use pyro. They have a door at the top of the soundstage around the ceiling that they keep open at all times during the show where the smoke escapes. As soon as the pyro goes off, the smoke gets sucked out of that door. So, it's possible. I don't think that Universal would put in an effect that would lead to it being taken out after a few weeks. They would have to get any flame gag approved first by a fire safety crew. Just my opinion.
I too see the flame either being in the Orfanage or at the entrance to Hades.
Also, open flame doesn't mean big huge fire. It could be a small flame or a big flame. Size wasn't specified.