Is there a hidden dhikr that no one knows except Allah, not even the angels, and has anything concerning it been mentioned in the Prophetic Sunnah?

The narration reported from Aisha (may Allah be pleased with her), with the wording: "The Messenger of Allah (peace be upon him) preferred the hidden dhikr (remembrance of Allah) that the guardian angels do not hear by seventy-fold..." is very weak, due to Mu'awiyah ibn Yahya As-Sadafi, who is very weak in hadith (definition). It was also narrated from her as a mawquf (وقف - a narration attributed to a Companion and not directly to the Prophet) with similar wordings, but with weak chains of transmission as well. Thus, there is nothing to prove a specific virtue for a particular type of dhikr called "the hidden dhikr."