This has bothered me for years. The previous owner somehow melted the wax ring under this toilet. The wax flowed in the grout between the tiles near the rear of the toilet and discolored/darkened the grout.
How can I get the wax out of the grout? I tried cleaning compounds without results. I am afraid, if I use paint thinner or Acetone, the wax will spread further, making things worse.
Since the affected grout is darkened, any future buyer will suspect that there is a leak under the toilet. (Just like wax, water also makes grout look dark.)