Yes, to me it looks like basically reflection glossiness map.
Some extra ideas:
1) In real life it is a layer of water on a reflective surface. So maybe layered mtl with mirror material and glossy material blended using the "steam" map would be a good solution. This way you would get both the sharp reflections of the mirror, and glossy reflections of the steam visible in some places.
2) Mirrors are usually covered with a layer of glass, so it might be good to model this. This is clearly visible in the heart example, where the reflections are doubled. This could add extra realism.
