What is CCleaner for Android referring to by "Hidden Cache"?
Other apps mention that as well, e.g. the cleaners of Avast and AVG – so it seems this isn't specific to the app. I would guess they refer to the apps' "private cache" (i.e. a cache directory inside the apps' private space in /data/data/&lt;package_name&gt;).

This developer article might give some insight. Quoting (emphasis mine):


  By default, Android Extensions adds a hidden cache function and a storage field to each container (Activity, Fragment, View or a LayoutContainer implementation) written in Kotlin. The method is pretty small so it does not increase the size of APK much.


Another insight from our sister site:


  … but other then that it's all saved on hidden cache partition.


Which suggests there's some special hidden cache partition somewhere – or the writer of that answer got it a bit wrong (which I think is the case).

A third variant: Some apps create "hidden cache folders" on the SD card. Now guess what "hidden" means here? sdcard/Android/data/&lt;some_app&gt;/.cache – a hidden directory (leading dot in the name) called .cache.

No straight answer, sorry – but my guess is it's referring either to the first or to the last of these 3 options. Highest likelyness is the latter – which seems to be confirmed by the SD Maid wiki¹:


  "Hidden caches" targets cache like files that are in non-default cache directories, e.g. videoCache, .cache.


One thing for sure: they're NOT talking about geo-caching with that 



¹ thanks to SSS for this pointer!