I was a spooky family mystery. There are records out there, particularly for the US, but you have think laterally and use resources outside their walled garden. US census records are only released after (IIRC) 70 years, so getting the 1950 census was a pretty big development. Beyond that, there are obituaries, phone books, newspapers, yearbooks, and others. I can even say from experience that the creepy "Radaris" style sites are usually leveraging some kernels of valid information in the free teaser data they show.