Leak in pond

I have a leak in my small pond ( 25'X14' aprox.) In the past I have allways been able to find them. Pond is banked with soil.

Where I see it comming out is a way from the pond---in the ground below ( no way to trace it back ). I can see it start slowly, but as pond fills up the water comming out way below increases. I think the leak is by the drain pipe, but the water on ground is 10' away.
This leak got me baffeled.

The pond Drained ↓

Attachment 2696



Any tips on how to find this leak.