One of the simplest approach ...
Published by Muhammad Alfalah Fauzi, Product Manager at Itron Inc
One of the simplest approach is that you can use a portable ultrasonic leak detector and use it at night along with step test method.
The principle is that you close the outlet of a section of a pipe network and check on the inlet flowmeter whether a flow exists or not. If flow exist, you can track the pipe using the leak detector.