Have you checked receiver settings screen on KK2 and set trim or subtrim on tx to center at 0 (1500ms) ?
Can you trim the yaw drift out or does it come back over time?
Have you run acc calibration recently?
Could one of the motors/esc be slowing which would cause the others to compensate and potentially get some yaw drift, check all motors spin freely unpowered, and bearings all feel smooth (no rough or notchiness, which would heat up in the air and bind)?
Lastly it could be vibration affecting yaw sensor, is everything (props/motors) balanced and running smoothly?
I am having a similar issue on my quad running Betaflight. ...
Edit, this happens even when not flying, hooked up to USB. About 1 degree every 3 seconds.
If it yaws on the bench it can only be gyro calibration or interference issue I'd think, I would recalibrate, and maybe try to move away from anything electronic which may be causing interference when calibrating (obviously as far as possible with cables etc).
Failing that its possibly a faulty FC with a drifting gyro (is it a Naze32 or ??), as if you calibrate and yaw drifts on the bench with nothing running there aren't many other options, I've not had a bad gyro on a board but have read others have so thats definitely possible
You could try to trim it out, but I suspect you would just keep chasing it if its a gyro issue, so probably best to get to the root cause and fix/replace.