Cleaning bathroom tile stains
I am renting. My bathroom has nice gray-ish floor tiles but the previous tenant left some yellow stains on them. These stains are right in front of shower, toilet and sink and seem roughly rectangular in shape. This is very suggestive that they formed under some rugs that were there for a while. I have no idea of their composition but these are very resistant stains. I tried everything under the sun and they are still there. I tried water, soap, 409, citric acid, Tide detergent, OxyBoost, almost all spot cleaners at Ace Hardware and Home Depot (some generic ones like ZEP, others with esoteric names I cannot remember).
My question is: what is the likely chemical composition of those stains and what is likely to get them out short of re-tiling the bathroom?