A plane that size hitting the surface of the ocean could have the impact of a detonation of, let’s say, 5,000 lbs of dynamite. The shock wave should have been detected. Surface ships, subs, listening devices.. something should have heard the crash. Information from several devices should allow for a triangulation of at least a general area. To give a sort of junior example, most cities now have microphones listening for gunshots. If a shot goes off, the data from several listening posts is used to triangulate a location and the cops are there right away. Yes, the ocean is bigger but sound travels through water immense distances.
Well, at the time there was (Chinese?) detection of some seismic event close to the “turning” point. That was dismissed. As for the open ocean, that is thousands of mi away from land and possibly many miles to any craft that would have any such capability or desire to use it. One also does not know just how much impact the craft actually would have - it could be slow and make a rather nice acute angle into the water, with less catastrophic impact than a headlong rush or somersault into it. That changes momentum and impact. (And bear in mind that aircraft are built to be light as possible.)
What happened with Air France? It was in the open Atlantic. Don’t recall if anyone talked about getting signals about shock or any such. And the Atlantic is narrower and probably more traveled by ships and US subs, especially where it was.
Not only that, weather satellites trained on the area watching cyclones form would catch the splashdown.