Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #10

فأما بعد انقراض أصحاب أبيه فقد كان مدة من الزمان أخباره واصلة من جهة السفراء الذين بينه وبين شيعته ويوثق بقولهم ويرجع إليهم لدينهم وأمانتهم وما اختصوا به من الدين والنزاهة وربما ذكرنا طرفا من أخبارهم فيما بعد.

After the companions of his father, communication was intact with him through the emissaries between him and his Shia, whose words were trusted and whose piety and trustworthiness made them reliable. We may bring some of their narratives in this regard in the future.