The modem recalibrates the noise floor periodically (every 2 s) from RX samples when idle. The receiver AGC is also reset periodically (every 30 s) so RSSI and noise floor do not drift to the minimum (-120). Typical range after calibration is about -120 to -90 dBm. Values may be 0 or briefly stale until the radio has been in receive mode long enough to collect 64 samples.
The modem recalibrates the noise floor every two seconds with an AGC reset every 30 seconds.