Well, having just listened to the entire tapes, WADS clearly weren't seeing anything either as they were relying on vectoring the f-15s with info from civilian pilots with visual identification of the unknown aircraft relayed through the civ ATC operators.

Presumably WADS has access to some of the most sophisticated radar equipment as isn't is part of NORAD?

The suggestion being no one was tracking this aircraft, not just a failure of civ primary radar.