Tā de xíngzōng shénchū-guǐmò, méi rén zhīdào tā zài nǎlǐ.
His whereabouts are unpredictable - nobody knows where he is.
Tips
memory
神 (spirit/god) + 出 (comes out) + 鬼 (ghost) + 没 (disappears). A spirit appears, then a ghost vanishes - together they describe something that materializes and dematerializes at will, like a supernatural being.
