Sometimes it deliberately wasn't kept secret from the enemy. This is from William Taubman's Khrushchev: The Man and His Era, about the Cuban missile crisis in 1962:
At 10:00 A.M., Washington time, when the quarantine went into full
effect, the U.S. strategic Command moved from Defense Condition 3 to
DEFCON 2, one level below that of general war. For the first time in
history all American long-range missiles and bombers were now on alert,
and scores of planes loaded with atomic bombs were aloft around the
clock, refueled by areal tankers, waiting over Greenland and northern
Canada for the signal to proceed toward the assigned Soviet target. To
make sure Moscow noticed, the SAC commander, General Thomas Power,
took it upon himself to "announce" the move in uncoded message to his
A footnote identifies the following source for the last sentence:
Laurence Chang and Peter Kornbluh, eds.,
The Cuban Missile Crisis, 1962: A National Security Archive Documents Reader
(New York, New Press, 1992), p. 371.
An "announcement" would also seem to make sense under the logic of nuclear deterrence. I can't confirm whether Power's communication contained the verbatim phrase DEFCON 2, but perhaps it did, because at this point (one hopes) nobody wanted to issue ambiguous commands.