Bandit,
It happens around ABZ all the time.The military are really good at steaming around 50ft below the base of the CTA,without talking.They usually squawk 7001 with mode c,but it's not verified.We have the base of the CTA, north and south down to 1500 ft and they still perform there even over high ground.
Their party piece is low level abort,into the CTA to terrain safe 4100'.Just where we are vectoring giant jetliners.

No warning,no 7700,just up they go in IMC.You file reports but they just melt away somewhere.