There are things that we just don’t know about search engines. Things that aren’t shared with us in an official blog post, or search engine representative speaker’s conference comment, or through a publicly published white paper. Often we do learn some aspects of how search engines work through patents, but the timing of those is controlled more by the US Patent and Trademark Office than by one of the search engines.
For example, back in 2003 Google was filing some of their first patents that identified changes to how their ranking algorithms worked, and among those was one with a name similar to the original Stanford PageRank patents filed by Lawrence Page. It has some hints about PageRank and Google’s link analysis that we haven’t officially seen before.
If you want a bit of a history lesson you can see the first couple of those PageRank patents at Method for scoring documents in a linked database (US Patent 6,799,176) and Method for node ranking in a linked database (US Patent 6,285,999).