Pee etches travertine. As does any acne medicine, or wine. Bath gels and body washes can stain. So can hair dye or toothpaste. You need larger samples to accurately assess the texture differences that you currently don’t see. You WILL see the duller eaten away spots in the context of a large amount of stone with smaller duller areas. Especially with repeat performances of pee and toilet cleaner around the bowl.

I will definitely try the toothpaste next. The pee -- my husband may thing I'm crazy but we will see LOL.

I don't think anyone will dye their hair in there. I have porcelain tile here at home and have not stained the tile (it comes off), but have badly stained the grout with my hair color.

I am also going to try sunscreen lotion.

Will update later!

My tile samples are about 6"x6". (I took before and after pictures of each section of tile I tested).

You need more like 24” of tile. As in, do a sample layout, with the grout. Lighting will make a difference too. Any bright glancing light will really highlight the etched sections.

We have honed travertine in two bathrooms (in one bathroom, the tile has a bit more sheen than the other, but it is honed, not polished travertine).

We've gotten etch marks in both bathrooms. When that happens, I take a green scrub sponge (sometimes dampened, sometimes dry if I’m lazy) and lightly “buff” away the etch mark. It works on the honed travertine, but don’t think it would do the trick on shiny, polished stone. Test it out on a sample tile or inconspicuous area first, to be sure you don’t create a bigger problem area.

I learned that trick here on GW. :)