How fast is your water going down?
If there is no sign of a leak outside the pond (wet spot of dirt), it could just be evaporation. If it's very hot and windy, evaporation can be quite high. You can test this out by filling a large container (or even not so large) and placing it in your pond so that the water level on your pond and in the container are the same level. If the level drops the same amount for the container and pond, then you know your water loss is just due to evaporation.