We've all been there, you have no idea how many times we've dealt with unexplained stains here, especially on carpets, and no matter how many times we explain that the stains are old, stuck in the padding, and reappear periodically, it's still considered our fault that they appear. From the location of the stains I would suggest that they are water stains, and maybe they have a small, barely noticeable leak behind their toilet. As for the cleaning, you could try diluted bleach, it's good for removing all kinds of stains from everything. And if your clients are not fans of bleach, you can try this lazy trick I have for stubborn stains, it's both efficient, nature friendly and doesn't leave an unpleasant smell. Just cover the stain with some baking soda, pour some white vinegar on top of it, leave it for at least 15 minutes to fizz, then wipe away. All the residue and stain should be gone, and if they appear again, then it's from something in their bathroom, not you.