I had the same thing happen and decided that it was some of the beauty products I was using. It is tough to trace because you use different products and then wash and dry your hands. My bath towels did not fade, but the hand towels did. My sons denture cleaner was also a culprit.
If things hang in the sun in the same way for a long time, they will fade too. Mostly that happens with curtains, but my shower curtain has done this.
Acne medicine, acne face wash, some types of speciality cleansers for the skin-- these things can cause fading, either from the handprints or from being wiped off, or on T-shirts, from sweating.
I've had that happen occasionally too. I think it might be bleach residue in the washer, if the last load I did was bleach. The water gets splashed around maybe the rinse doesn't get it all. Next load, towels that end up with fade marks.
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