Question

where are signed but unlogged caches

Answer 1

In your profile stats under the Challenges tab, if you have some. For example, you can see mine stats which have one:

https://project-gc.com/ProfileStats/Jakuje#Challenges

Answer 2

Signed but unlogged is where you have placed a write note log on a challenge cache (eg noting your qualifying details), but not yet logged a find.

Comments

No, that should be the other way round -- it should be the caches that you logged physically, but you do not qualify with the condition yet.

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Sorry, Jakuje, you are incorrect. PGC does not know that you have physically logged the cache. My reply is correct. I have challenges that I announced checkers on with a Write Note, in countries I have never visited, that PGC claims are signed but not logged as found.

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I know that many people do that this way, but it does not mean it is correct -- you can run the checker any time later and you usually get the same results. The "Write note" logs are for logging your visits (as the title says when you see the button on geocaching website) not for logging your intention to visit.
This is explicitly mentioned in the challenge rules, in point 7, Signing Log (you can visit the cache anytime, but logging found it only later):
https://www.geocaching.com/help/index.php?pg=kb.chapter&id=127&pgid=206#signingLog
The write note is the only way how the pgc can know that you visited the cache, but you did not log the found it yet because you do not fulfill the conditions.
Also the title in PGC stats confirms this:  Challenges signed (but not yet logged as found).
If one wishes to keep a list of challenges he fulfills or intends to visit, there are better tools than spamming the write notes -- we have bookmark lists and VGPS.

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Took some fiddling but finally found it.  I'm writing down the answer in several places so I can find it again.  The list I wanted and finally found are caches that I have physically found and signed the log BUT either do not qualify or need to pull together my qualifications [the pre-challenge checker caches].  Many thanks.