Some ADS-B receivers do that with GPS sync. If a position has poor accuracy and is shown as "mlat" it' multilaterated based on when multiple receivers received

Some ADS-B receivers do that with GPS sync. If a position has poor accuracy and is shown as "mlat" it' multilaterated based on when multiple receivers received the ADS-B packet. So while the plane is not transmitting it's location, the transmission can be used to identify and locating it. An actual passive radar network with multiple receive sites is reportedly a commerical product from multiple vendors, but it's not easy nor cheap.