ephy uses org.epiphany.FormPassword — not org.gnome.Epiphany.Whatever, which is hilarious because https://epiphany.org/ is... something entirely unrelated, but I guess this is way too late to change now. Other apps use their own things, it seems.
There's also org.gnome.keyring.Note defined by libsecret itself, which is kind of neutral and documented as being for "note or password stored manually by the user".