Just discovered this:
If you write an unlisted toot which contains a #-tag the web interface tells you that they will only be visible in public toots, however, you can discover them over the user's featured tags list or by adding /tagged/ and the #-tag.
For example all my #libremSocial-posts can be found here: https://social.librem.one/@bsp/tagged/libremsocial including the unlisted one above⬆️