What happened
This is an aggregate entry, not a single sighting. Leonard David reported in Space.com (September 2, 2004) that NIDS "has amassed almost 400 separate sightings of triangular/boomerang/wedge-shaped objects." Many describe large, silent, slow, low-flying craft. NIDS noted that sightings clustered along routes between Air Mobility Command and Air Force Materiel Command bases. In 2002 NIDS suggested the Illinois triangle was a DoD craft. In 2004 it said the craft's behaviour "does not appear consistent with the covert deployment of an advanced DoD aircraft."