You're probably looking for the OPDS browser. I don't use it, so I don't recall where exactly it's hidden. Cloud storage, maybe? (It requires Calibre's content server to be accessible, and possibly passwordless).
The "Find a book in calibre catalogue" option only concerns the on-device books, provided they were sent to the device via Calibre (over USB, I'm not sure the wireless client updates it). It is, fittingly, located in the search tab, as that's the gist of what it does

.