Mysterious

puzzling, strange, peculiar, curious, funny, odd, weird, bizarre, mystifying, inexplicable, baffling, perplexing, bewildering, confusing, uncanny, dark, impenetrable, incomprehensible, unexplainable, unfathomable, Delphic, sibylline, unaccountable, insoluble, obscure, arcane, recondite, secret, esoteric, occult, cryptic, hidden, concealed, supernatural, mystical

mys·te·ri·ous

adjective

difficult or impossible to understand, explain, or identify.

"his colleague had vanished in mysterious circumstances"