When I dug my big pond, I hit an area of sand that was probably about a thousand square feet. I dug it out several feet to where I couldn't find any more sign of it along the edges, and then filled the hole with re clay from other parts of the pond bottom. All it takes is a small area for the water to leak out, but the only way to know where it is and how big it is will be to start digging and examining the material that comes out.
Yeah, I am sure you are right. For now I need to get through the rainy season so I can dry it up enough to go digging...can't wait!