Yes, even thermal background noise (the noise level that exists even in complete absence of RF) would be expected to be above -140dBm. It scales with channel size and temperature.

As near as I can tell, the smallest subcarrier 5G can use is 15kHz, the thermal noise floor for a 15kHz channel at room temp (300K) would be -132 dBm.

My guess is whatever chip doing the measurement simply couldn't measure that low accurately, or it reports "nothing detected" as -140 dBm.

GPS receivers can go down to around -160dBm, but that's a very low bitrate signal. Nonetheless it is possible to receive such signals via CDMA techniques.

Reminds me of "The Hunt For Red October" and the sonar consoles "magma displacement" reportings...