There are several reasons why ponds 'lose' water without there necessarily being an actual structural leak. These should all be investigated before the lengthy - and messy - process of leak repairs gets underway. If you can eliminate the following, then - and only then - your should prepare for repairing a leak in your pond.
Evaporation. Substantial water reduction can take place - in a short time - simply by the process of evaporation of the pond's surface.
Drying winds wafting across the surface can lead to a significant water loss - resulting in a lowering of the pond water. Evaporation is further increased with the use of fountains and waterfalls.
If the pond surface remains at a constant level when waterfall or fountain is not running, then these can be suspected as the primary cause.
Waterfalls are often the culprit with a noticeable loss of water in the pond. Any spillage from waterfalls should be noticed fairly easily if it is seeping over the sides. However, if the water is leaking through a joint in concrete and stone fall, then it might be more difficult to spot if the water is seeping through the bottom of a fall structure.
Stop the fall for a few days and firstly make sure that intermediate pools are not leaking. If these are ok - and the main pond area seems stable, then suspect leakage from between the intermediate pools and in the structure of the fall itself.