We are talking about leakage ...

We are talking about leakage in water distribution systems and a major issue is small, nonsurfacing leaks that do not produce a significant pressure differential.  Because these leaks can run for months, water loss is significant (and some eventually cause damage when the leak evolves into a major break or undermines the soil)  For nonplastic distribution pipe, correlating acoustic monitoring is a well practiced technique available from several vendors (including Echologics, Guterman, Aquarius Spectrum).