I have a claim open at the moment, very similar. 15+ year old nylon, and mysterious, and random sized and placed colour loss throughout the entire home.
I inspected it late last year for the insurer and it's not unlike yours, older couple living in a very clean home, no cleaning person coming in. No consistency in shape or location of spots. No urine evident to either smell, or UV black light. No teenagers, no acne or foot treatments.
The rooms with more natural light, are worse affected, so my initial dip educations was a deliberately used spot cleaner containing a low percentage 'agent' in it that is being accelerated by natural light and compromising the integrity and stability of the dyes.
They clean their carpet themselves annually, usually with a rug doctor and then spot regularly themselves. They have been using the rug doctor spot cleaner for spotting during the year. They didn't have a bottle in the home at the time, but it's readily available if I want to conduct either tests myself, or look for similar residues for comparative analysis.
I have just had the insurer come back to me requesting two things. First, conduct further testing to see if I can narrow down a likely active ingredient and therefore look for likely products causing this damage (so it can stop being used) and secondly, they want me to see if I can neutralise the active indigent in a few sample spots and re dye them. I get the feeling they intend sitting on it for a few months to see if it reoccurs. I told them if I proceed with the re dye, it may not be successful long term, and may reoccur.
John
