NIGHT WATCH / NAOC
Night Watch
or Nightwatch
is the program identifier for the USAF National Airborne Operations Center (NAOC), including at least certain Boeing E-4 flights.
NAOC
(used in flight):
From JO 7110.65W:
The term “NAOC” will not be a part of the call sign but may
be used when the aircraft is airborne to indicate a request
for special handling.
FLYNET
From JO 7110.65W:
The code name “FLYNET” indicates that an aircraft is
transporting a nuclear emergency team or a disaster
control team to the location of a potential or actual nuclear
accident or an accident involving chemical agents or
hazardous materials. It is in the public interest that they
reach their destination as rapidly as possible.
GARDEN PLOT
Classified. Related to operation Garden Plot. Presumably a callsign to be used by aircraft as part of that operation.
SCOOT
Term related to Special Air Mission flights in evacuation of government VIPs.
From JO 7110.65W:
The term “SCOOT” will not be part of the call sign but may
be used when the aircraft is airborne to indicate a request
for special handling.
OPEN SKIES
Callsign for observation aircraft under the Treaty on Open Skies. The USAF aircraft used for this purpose is the OC-135.
From Wikipedia's Treaty on Open Skies:
Observation/Demonstration flights are conducted
under rigid guidelines outlined in the Treaty of OPEN
SKIES that govern sensor usage, maximum flight
distances, altitudes and priorities.