https://catalog.altadenalibrary.org/cgi-bin/koha/opac-detail.pl?biblionumber=162348&query_desc=

If the SM58 noise floor is calculated at room temperature, the voltage output is 0.00000032 volts.

This is equal to -130 dBV. The SM58 Vocal Microphone Specification Sheet says:

Sensitivity (at 1,000 Hz Open Circuit Voltage) –54.5 dBV/Pa (1.85 mV)

At this sensitivity, the self-noise would be 94 - (-54.5 - -130) = 19 dB SPL, which is pretty typical and certainly not "lower than can be typically measured".