We are surrounded by ghosts

I'd like to call the more general phenomenon that this is a specific instance of "ghost knowledge": It is knowledge that is present somewhere in the epistemic community, and is perhaps readily accessible to some central member of that community, but it is not really written down anywhere and it's not clear how to access it. Roughly what makes something ghost knowledge is two things:

  1. It is readily discoverable if you have trusted access to expert members of the community.
  2. It is almost completely inaccessible if you are not.

In this sense, most knowledge is ghost, particularly if you take an expansive view of what counts as an epistemic community.