神出鬼没 | shén chū guǐ mò | lit. to appear and disappear unpredictably like a spirit or a ghost (idiom) / fig. elusive | |
变幻莫测 | biàn huàn mò cè | to change unpredictably / unpredictable / erratic / treacherous | |
出没无常 | chū mò wú cháng | to appear and disappear unpredictably |