If you can heat the water (or in some cases cool it) so it has a significant different temperature from the surrounding you can use thermography. Done right the pipes will show up and a leak will show up as some cloud near a pipe.
Another one is using a gas (5% hydrogen, 95% nitrogen, plus some foul smelling additives) and a gas leak detector. Requires you can empty the pipe system, close all taps, etc. Pump the gas in, use the gas leak detector to detect where the gas is coming out.
For large pipes you could try a camera inside the pipe.
There is also an industrial technology using microwaves. Microwaves are used to measure the wetness / dampness of materials. The surrounding of the leak will show up as more damp.
Acoustic techniques have already been mentioned. A simple stethoscope can sometimes help to find a larger leak. High sensitive microphone to find smaller ones. They typically require that you have a pretty good idea where to listen.