You are correct in that using a drum sander is not at all DIY friendly. But it's the only way to correctly sand a floor down to smooth and ready to finish. You will take years trying to do the same thing with an orbital. The rental rate for the sander is going to be more than hiring the right pro with the right equipment.

I don’t have personal experience with that like I do Schluter, but I bet one of the industry pros has. Do you have pictures of the start to finish build? I hate to second guess an unknown professional’s work, but why would they want to mix systems? Because there is now no manufacturer’s warranty, except what the tile guy gives you. How long is that? 1 year? How long has he been business? How long had his references had showers done? Years? Plus, the interface between two different systems is the weak point, and there is no accepted written protocol ever to mix systems. It could be fine. Frankenshowers often are fine. But they rest soley on the shoulders of the installer. Everything is about the installer, rather than the manufacturer’s system and installer. At least verify that they followed the installation instructions of this part of their self created system. https://www.jameshardie.com/JamesHardieNorthAmerica/media/Products/Hydro/HydroDefense_Install_Instructions_English.pdf

If you can't feel anything, then it's probably not removable, but glass that's been scorched, as millworkman says. I had that happen on my first glass top stove from using an aluminum steam canner on it. I think it was partially the high heat and the aluminum combo. When I got a new stove I also bought a hotplate to use with the canner and I've had no more problems. You really need to keep the stovetop free of spills and use special cleaners so they don't scorch. I clean mine practically after every use, because something always seems to spill or dribble. Look at youtube about how to scrape off burned on food with a razor blade. If anything will come off, that will do it, but you have to use it properly or you'll scratch the glass. I use ceramabrite or Sears glass top cleaner. You can also try soaking it with distilled white vinegar (let it set a long time, then sprinkle with baking soda to see if you can scrub any of it it off) to remove any minerals from water that may have hardened on it. If you can't get it off, you might see about replacing the glass top instead of being charged for a new stove. I cracked a glass top once and replacing the glass was $$$ but about 1/2 the price of replacing the whole stove. https://www.youtube.com/watch?v=MnvBrMehn_k

Thank you for the comments so far! To be clear it was not a refinishing. The floor was put down and finished by the same contractor, meaning this is the "original" sanding. Would you live with it? Given it's been 2 years already I am thinking we don't do anything until we re-sand. When we do (re-sand) will it be hard for the new contractor to fix it? I'll ask about hardplate (and I also read about oscillating sander?). It's quite odd that we only see the marks in one area of around 10% of total surface area...perhaps due to the light in that area only. Moving forward I guess my main worry is that the next sanding in a few years makes this worse and not better. For the life of me I cannot see a difference in height and was hoping you'd all tell me it was either the way stain was applied, or the poly. Also, @Sye Phommachanh what are "dishouts"? thank you!