Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #12

وهذا الخضرعليه السلام موجود قبل زماننا من عهد موسى عليه السلام عند أكثر الأمة وإلى وقتنا هذا باتفاق أهل السير؛ لا يعرف مستقره ولا يعرف أحد له أصحابا إلا ما جاء به القرآن من قصته مع موسى عليه السلام. وما يذكره بعض الناس أنه يظهر أحيانا ولا يعرف ويظن من يراه أنه بعض الزهاد فإذا فارق مكانه توهمه المسمى بالخضر ولم يكن عرفه بعينه في الحال ولا ظنه فيها بل اعتقد أنه بعض أهل الزمان.

Khizr (a.s) is alive since before our time, since the time of Musa (a.s) according to the majority of the Ummah until our time. It is a matter of consensus amongst historians that no one knows his place of residence and no one knows if he has any companions, except his story with Musa in the Quran and the various narratives that some people think they have seen him as a pious man and after separating from him realized that he was Khizr.