If the software could tell there is nothing interesting in a WU in a few minutes it would.

The only thing the software can tell in a few minutes is silence (these are probably filtered before being sent out), or a drowning excess of strong (man-made) signals.

Other than that, (over simplified) the software needs to laboriously compare numerous 'pure' frequencies with the recorded white-noise to see if those frequencies exist in the noise over a period. AFAIK there's no known short-cut that hasn't already been incorporated.

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