The aborted [fetus] because it is born before its due time, is immediately concealed [as] dead.
-Moralia, Bk. IV, line 3 [private translation]
Moreover this abortion is best said to be secret because from the origins of the world, when we know Moses was writing certain things, part of the human family was greatly unknown/concealed to us.
-Moralia, line 25 [private translation]