You quoted Grammar Girl as saying this:
The reason I didn't say that I absolutely should have hyphenated "noise canceling headphones" is that if leaving out the hyphen causes no ambiguity, some style guides, such as the Chicago Manual of Style, say it's OK to leave it out; and I don't think anyone would read my meaning differently with or without a hyphen.
This is located at http://www.quickanddirtytips.com/education/grammar/how-to-use-a-hyphen?page=1
GRAMMAR GIRL GOT THAT ONE WRONG!
CMOS definitely does not support her contention that it is "OK" to omit a hyphen from 'noise-canceling headphones'.
[Note, I'm not claiming to know more about grammar than Grammar Girl. I'm claiming I have the book she quoted in front of me, and her interpretation of one section, 7.80, is incorrect, because she overlooked the fact that section is only relevant when the conditions listed in another section, 7.85, are not applicable.]
The relevant section in CMOS is 7.85, titled 'Hyphenation guide for compounds and words formed with prefixes'.
This section begins with an introduction to tables as follows (with some parts edited for simplicity)
When using this guide … consult the preceding paragraphs in this section (7.77–84)—especially if a relevant example cannot be found. In general, Chicago prefers a spare hyphenation style: if no suitable example or analogy can be found either in this section or in the dictionary, hyphenate only if doing so will aid readability. The first section (of the following table) deals with compounds according to category; the second section, with compounds according to parts of speech. The third section lists …
The 'noise-canceling' example is covered in the second section of the table with the entry for 'noun + participle'. CMOS recommendation 'hyphenated before a noun, otherwise open'. It provides an example of 'clothes-buying grandmother'. Note that there is no potential ambiguity with that phrase, yet CMOS still recommends the compound adjective should be hyphenated.
The quote in CMOS that Grammar Girl has misinterpreted appears to be 7.80, titled 'Hyphens and readability'. It states:
A hyphen can make for easier reading by showing structure and, often, pronunciation. Words that might otherwise be misread, such as 're-creation' or 'co-op', should be hyphenated. Hyphens can also eliminate ambiguity. For example, the hyphen in much-needed clothing shows that the clothing is greatly needed rather than abundant and needed. Where no ambiguity could result, as in 'public welfare administration' or 'graduate student housing', hyphenation is unnecessary.
That section is horribly written, but it should not be applied unless there is no suitable example in 7.85. One situation which has no example in 7.85 is with two nouns modifying another noun. There may be others, but that is the form of both examples they provided. Note that those examples do not have a compound adjective. Both 'welfare administration' and 'student housing' make sense on their own, and they are merely being further modified when the 'public' and 'graduate' are added in front of them. That is not so with the example she was considering, nor the one we have been discussing. Neither 'canceling headphones' nor 'known archetypes' make sense.