Look at them in sequence and you will see teh SNR line in reasonable "smooth" up until the 21:00 image where it starts to become a lot rougher and more so at 22:00 and 23:00. That, to me suggests pick up from radio waves and look, for example, how the notch at tone 602 (2.5-2.6MHz) gets significantly worse.
The real oddity, still, is the total lack of any usable tones from 300 to 520 ...
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taurus excreta cerebrum vincit