That's interesting. I've noticed the same thing. It's not a ring, but I've noticed patches of orangey type stuff that wipes off very easily in my tub too.
Never thought much of it. I assumed it was something in the water reacting with the tub or the tiles.
It's usually more noticable in the drain area, but I see a little bit on the sides of the tub. I only take short showers, and use a shower filter as well.
Interesting. I wonder if this is the same thing you speak of.