https://en.wikipedia.org/wiki/Cross-correlation

this could be sound from loudspeakers or broadcast audiorate electromagnetic waves getting picked up in the audio cables, or even a passive system of sensors throughout russia, i.e. using cross-correlation with background noise ...

one could also view this as a pseudorandom form of lock-in amplification, and even with a high noise floor the results do improve with number of samples, so patience is all they need.