Julius Reizen in reviewing over Umberto Eco's index card system in How to Write a Thesis, defines his own "insurance card" as one which contains "hard-to-classify thoughts, questions, and areas for further inquiry". These he would keep together so that they don't otherwise get lost in the variety of other locations one might keep them

These might be akin to Ahrens' "fleeting notes" but are ones which may not easily or even immediately be converted in to "permanent notes" for one's zettelkasten. However, given their mission critical importance, they may be some of the most important cards in one's repository.

link this to - idea of centralizing one's note taking practice to a single location

Is this idea in Eco's book and Reizen is the one that gives it a name since some of the other categories have names? (examples: bibliographic index cards, reading index cards (aka literature notes), cards for themes, author index cards, quote index cards, idea index cards, connection cards). Were these "officially" named and categorized by Eco?

May be worthwhile to create a grid of these naming systems and uses amongst some of the broader note taking methods. Where are they similar, where do they differ?

Multi-search tools that have full access to multiple trusted data stores (ostensibly personal ones across notebooks, hard drives, social media services, etc.) could potentially solve the problem of needing to remember where you noted something.

Currently, in the social media space especially, this is not a realized service.

https://web.archive.org/web/20120122115952/http://pileofindexcards.org/blog/2006/10/13/one-pocket-rule/

Noguchi Yukio had a "one pocket rule" which they first described in “「超」整理法 (cho seiri ho)”. The broad idea was to store everything in one place as a means of saving time by not needing to search in multiple repositories for the thing you were hunting for. Despite this advice the Noguchi Filing System didn't take complete advantage of this as one would likely have both a "home" and an "office" system, thus creating two pockets, a problem that exists in an analog world, but which can be mitigated in a digital one.

The one pocket rule can be seen in the IndieWeb principles of owning all your own data on your own website and syndicating out from there. Your single website has the entire store of all your material which makes search much easier. You don't need to recall which platform (Twitter, Facebook, Instagram, et al.) you posted something on, you can save time and find the thing much more quickly by searching one place.

This principle also applies to zettelkasten and commonplace books (well indexed), which allow you to find the data or information you put into them quickly and easily.

https://searx.github.io/searx/

Mentioned by Taylor Jadin.

https://warmedal.se/~bjorn/posts/2022-07-02-providing-meaningful-search-results-without-own-index.html

https://context.center/topics/indie-search/

http://oldavista.com/

Old'aVista, a play on the old Altavista search engine.

https://search.marginalia.nu/

A search engine for IndieWeb sites.

https://indieweb-search.jamesg.blog/about

They certainly could. I've seen the author haunting the IndieWeb chat in the past and they've mentioned that crawling and saving data can tend to be a bit on the expensive side, so they're trying to do more targeted search/save when they can. Perhaps as the project matures, it will add these sorts of functionalities.