Once the leak is found the liner can either be patched or a new liner installed. A little leak is one that goes down less than an inch a day and may not be worth repairing. In fact, a slow leak can simply keep the plants watered around the pond. A more serious leak, however, can mean tragedy if the pond leaks dry and cannot be ignored for long. It could be splash from an over-zealous pumping system.
Most leaks occur in the waterfall or streambed.
Reroute the water if you can so that it bypasses the waterfall. Note the water level. Let the pump run all night then notice if you've lost any water. If you haven't, you know the leak is in the waterfall. If you can't bypass the waterfall turn off the pump.