"Somebody would have noticed" is shorthand for a certain argument. Like most shorthand arguments, it can be used well or badly. Using a shorthand argument badly is what we mean by a "fallacy".

A shorthand argument is used well, in my opinion, just if you could expand it to the longhand form and it would still work. That's not a requirement to always do the full expansion. You don't have to expand it each time, nor have 100% confidence of success, nor expand the whole thing if it's long or boring. But expanding it has to be a real option.

Critical questions that arise in expanding this particular argument:

• What constitutes noticing?

  • Would other people who noticed understand what they saw?
  • Further, would they understand it the same way that we do?
    • How much potential is there for their understanding of the same phenomenon to be quite different from ours?
  • Further, if their understanding is similar to ours, would they express it in terms that we would recognize?
    • This could include actions that we recognize as relating to the phenomenon.

• Would we know that they noticed?

  • Motivations: Would people who noticed have strong motivations for letting us know or for not letting others know?
    • Would they want others to see that they noticed?
    • Would they want others to see the phenomenon they noticed?
    • Would they want to do something about it that someone could easily see?
  • Ability:
    • If they did want others to know, could they easily show it?
    • Conversely, if they didn't, could they easily hide it?
  • Who witnesses it:
    • Would they want us in particular to see it (or not see it), as opposed to a select group? For instance, they might write a report about it that you and I probably wouldn't see.
    • If they revealed it to others but not directly to us, what's the likelihood that the information would make its way to us?

• The suppressed premise in that emthymeme is that "Nobody noticed". Since we didn't ask everyone in the world, how did we determine that?

  • What is the population that would have noticed?
  • What sample size did we take?
  • How representative was our sampling?
  • Assuming we have reasonable answers to the above, what level of confidence can we place on our sampling?