Thanks, that was very helpful to add the additional information.
It does sound like a probable leak at this point.
Next test for you: After you get some significant rains and the pond is mostly full, wait until the "normal" ground on your property has dried out, and THEN do a good walking survey around your pond. Look for standing water, damp soil, or even a change in plant species relative to the adjacent areas at slightly higher elevations. That will help confirm a leak and start to give you some assistance in determining its location.
