The reality is somewhat more complicated than that. Signal processing at this level is statistical in nature, so the threshold itself is effectively timing-driven.
Hard to explain in a few words, but the principle of the lock-in amplifier is a good place to start.
I still don't understand how there is no threshold based on amplitude (correlation or not). Something like mist would show up as a noise floor, a speck of dust (that hangs around for long enough) may be one peak, a mirror another, and an object seen in the mirror will be another. You will have multiple peaks that may stay around longer than others and, but there will still be peaks, one of them being the "true" surface.
Hard to explain in a few words, but the principle of the lock-in amplifier is a good place to start.