Probably not an overflow valve. There could be a spillway somewhere, but that isn't your issue now. Leaks and unintentional overflows are often hard to find. A picture might be of use and save us all some writing (or not). Before knowing anything else, can you turn off the pump and see if the pond stops losing any or as much water? That will be a good start. Where are you.

I will try and post a pic tomorrow. We had shut off the water fall and it made no difference. It seems to me that the water level drops because the water outlet or intake into the filter is set so low into the side of the pond. The water runs out to that level and then stops. We tried blocking it partly up today with some success. I am at work now but I will check it when I get home. We are in the Black Hills of South Dakota.