2) Any trees or significant vegetation growing on the dam?
3) Any signs of animal damage? tunneling, burrowing, etc.
4) Have you tried an evaporation test? put pond water in a 5 gallon bucket next to a stake in the ground marking the water level of the pond...wait a few days(without rain) and measure the drop in water line in the bucket and the vertical drop in the pond from the stake. If they are about the same, then evaporation...if the pond level drop is significantly greater then leak likely.
Loc: Grayson County
I second Meadowlark, but also would inquiry about the soil type. Sand, clay, loamy, caliche, etc. This also helps to determine a possible reason or cause, help get you on track to what might be the cause.
Most ponds and stock tanks will have some water seepage if the dam was not properly compacted, even with good clay. When the pond is full, hydrostatic pressure is greatest and leaking is most prevalent. Sometimes, the water level will drop to a point where the seepage will be negligible or stop.