" i.e. it is a term for something that cannot be found easily
using the Google Search web search engine. It is increasingly used to mean something that cannot be found using any web search
I've brought this concern to attention once before. There are many circumstances I never even try to lookup something using a search engine, despite the possibility (even likelihood) the subject may have sufficient information regarding it published somewhere on the web (or implicitly within files/data indicated using W3C standards).
How does DuckDuckGo conduct its trial/quality investigation? Consider funding real work
which may require intensive usage of searching through information
-- whether by books at a library or however they manage to succeed most efficiently -- after designing a rigid way to measure their performance and its relation with normative web search engines (whether they use DDG, Yahoo, Google etc)
You might even want to employ a scientific team to progress the fidelity of this investigation in a dedicated, evolving series of trials. Be brutal with your investigation. Bootstrap
All web search engines are quite useless to me (can't even express query) in most cases when they SHOULD NOT be.