How does FR find out when a Freeper has passed?
No idea.
Ask silent_jonny. He made that page.
Usually someone who had ongoing personal communication with the Freeper that's passed, posts it in the forum. That is, if someone from the family lets them know of the passing. I was communicating with a long-time Freeper up until last fall. Last communication I had from her, she was in the hospital, then nothing. I wrote her via FR several times but got no replies from her or a family member, and I had no personal information to check on her. It was another Freeper who posted her death on the site.