Gyro calibration offset

Looking in
https://github.com/PX4/PX4-Autopilot/blob/master/src/lib/sensor_calibration/Gyroscope.hpp it seems to be the case that the calibration used during flight compensates for an offset. As I understand it by looking in https://github.com/PX4/PX4-Autopilot/blob/master/src/modules/gyro_calibration/GyroCalibration.cpp the offset is the gyro bias during the time of the calibration process.

Questions:

  1. Is the calibration always performed prior flight/arming (bias at start up will most likely never be the same for a MEMS)?
  2. Are there any other processes/tasks that might set/change the offset during flight?
  3. If the offset might change during flight, how is that handled by estimators (EKF2) down the line?