In a scan-set of utility frequencies, there are daytime freqs and night time
freqs.

This wasn't what I was talking about.

Yes, DOD, CG and other HF users respond to different PROPAGATION
characteristics by having "day" and "night" channels. What I was referring to
was that the noise level ("noise floor") changes over time.

For example on 11175 (which is a day AND night channel) I'd have to adjust the
threshhold as the day went on, because an increase in background noise
continued until after dusk. And then around midnight, the noise floor would
drop and world-wide signals would come in. With my R71, I'd make two or three
adjustments a day, at least.

The RF 505 NEVER needed adjustment because it "sensed" voice rather than
"sound". It, and my Scientific Radio Systems fixed-channel radios, hardly ever
responded to lightning. The Icom would be triggered all the time.