C. Wright Mills' idea of "fringe-thoughts" is similar to Ahrens framing of "fleeting notes".

Goutor defines self-help notes as notes which one would use to refresh their memory about what remains to be done or researched, problems that remain to be solved, or information which is needed to be researched or found. (p26) These are akin in some sense to what I call "open questions". He also indicates that these notes might be triggered by one's daily activities or occasional musings which relate to one's project but occur outside of its active pursuit. In this sense, they have a similar feel to the idea of Ahrens' fleeting notes, but in Goutor's practice they aren't defined as occurring while one is doing active reading or research.

He suggests that one keeps these notes in a separate area so that they might be systematically and regularly visited for review, further research, or answering as the opportunities to do so present themselves. Once the questions have been answered and appropriate notes updated or added, these self-help notes can be discarded.

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.

While one can certainly capture a lot of cruft that isn't actionable or easily reusable, it's a fable that the best ideas will come back around. All too often the really brilliant ideas can be quickly lost to the wind if not captured immediately.