My university recently discovered a covid outbreak at my freshman dorm by testing the wastewater; that was the first I heard about the technique. It's especially effective for this use case: "Traces of SARS-CoV-2 can show up in an infected person’s feces days before any symptoms are detected, and even when a person ultimately shows no symptoms at all." [1]

[1] https://adminvc.ucla.edu/expanded-wastewater-testing-program....