This discussion has come up before, so I thought this article from PMG might interest some of you. https://www.pmgnotes.com/news/artic...detection-dollar-bill-missing-serial-numbers/
Perhaps they noticed the disturbed paper surface where the serial numbers were erased. EDIT: I see they think it was a chemical alteration. I vote both. Chemicals don't damaged the paper as this shows. PS But what do I know, I'm just a coinhead.
This link, along with the other links in the articles, will be printed out and put in my notebook. Thanks for posting the link.