“Accounts of interaction between air traffic control (ATC) radar operators and SR-71’s are manifestly apocryphal. ATC radar won’t track targets moving at 4000 kt.”
Mach 3 is 2000 knots
Still outside the range of ATC radar. The point is that radar sets do not track objects outside the performance envelope they are designed for. Fighter aircraft’s airborne radar is highly optimized to target other warplanes, “UFOs” would require IF bandwidths and tracking filter process noise that would degrade performance against targets of interest.