You flew your drone almost 4000' feet away from you at low altitude over the ocean. Pretty good chance you couldn't see it at all. Maybe a wind gust. Maybe a bird strike. Maybe an angry mermaid. Almost anything could have happened. No person can...
Not true, the Mavic Pro has a precision landing system. It just isn’t as developed as later drones but with a landing pad was certainly capable of landing within inches of its takeoff point.