From the report:
With 4 crew members able to conduct lookout from the 2 F15 aircraft, military members opined that they should have been able to visually sight the Voyager, despite its grey camouflage scheme, as they climbed towards it. In both these respects, Board members wondered if the F15 crews had become task focused and whether this may have been the reason that they missed or discounted the Voyager both visually and on their radars.
Which would indicate that the F-15s were indeed Mud Hens.
The likelihood of a Voyager NOT being seen on 'state of the art' radar is zero - unless it has a Klingon cloaking device activated.... The report states that the F-15 pilots were 'flying visually'; the field of view from an F-15 is excellent, so it remains a mystery as to how the F-15s didn't spot the Voyager in 10km plus visibility.