Depending on how serious you are (but maybe not "police" serious) and also your financial situation, this is the kind of stuff a Private Investigator can do.

My first thought is contact police because only they have the resources to start an investigation, looking into phone data and thoroughly searching.

My second thought is if you know someone with knowledge or if you have time to read up on OSINT skills, these could help if you need to do your own search.

Maybe someone has better ideas though.

If none of the online and similar stuff works, it might be time for the family to file a missing person report. See https://en.wikipedia.org/wiki/Philip_Agre about a guy who disappeared like that. He was eventually confirmed to be ok, but had apparently decided to go off the map for personal reasons.

There was also a medical researcher whose name I've forgotten, who also went dark iike that. Her family eventually found her and she said oh, she hadn't been up to much, and just didn't realize that anyone was looking for her. She did some well known work connecting allergies with cancer but I haven't been able to identify her through a few web searches. Anyone remember?

Added: aha, found. https://en.wikipedia.org/wiki/Margie_Profet

See also: https://www.psychologytoday.com/us/articles/201205/the-mysterious-case-the-vanishing-genius

Added 2: Oh my, I didn't know the whole story. Profet apparently had physical and mental health issues, possibly schizophrenia, and she gradually pulled away from people who knew her, rather than suddenly vanishing. After she had been missing for years, the article above found its way to her and she contacted her family. IDK what if anything she has been doing since then, but she appears to still be around. She reminds me a bit of John Forbes Nash, mathematician who was the subject of the book and movie "A Beautiful Mind". https://blogs.nature.com/news/2012/05/missing-biologist-surfaces-reunites-with-family.html