I would think it's just oils, lotions, etc coming off of the bathers in your tub. I get a ring around my tub every once in awhile as well. Mainly only after I have company in the tub, typically female. They make products such as Defender from Liesure Time that help break up the matter before it causes a ring. There are also these things called scum balls, that float around in your tub, that capture the oils. I personally just scrub it off with a Magic Eraser after I add a dose of dichlor. This is one of a few reasons why I shower 95% of the time before I soak.
.......................Or it could just be a hard water stain? :-/