I was burning a cone incense and left it on the sink, and this stain won't come off. I tried baking soda, water, vinegar, hydrogen peroxide. I tried letting them sit for multiple interations, and it made no difference.
Your nice list doesn't seem to have non polar solvents in case it's greasy. Something like soap, alcohol, (turpentine, acetone, xylene).
But yeah, might not be a surface stain as the other comments suggest.