Carrier Sleuth generates very high resolution waterfalls from I/Q recording files made by several SDR (Software Defined Radio) programs. While you can often only hear one, or perhaps two, stations on a frequency at a time, chances are there are actually many other stations lurking on that frequency, even in the daytime. Perhaps dozens.
The problem is that they are too weak to notice using normal listening techniques. There's no audio, and even their carriera are often quite weak. But by performing very high resolution FFTs (Fast Fourier Transforms) on recordings made by SDRs (Software Defined Radios) you can extract these carriers up out of the noise and see them.