What will goldfish eat in an outside pond?

There is way of telling without actually witnessing what is happening. If you live where there are bird predators like kingfishers, cormorants, egrets or any of the many other water birds, they could be your problem. On the other hand if there are felines in the area they too could be removing fish from a garden pond. Depending on the size and shape of the pond maybe you could design and build some type of cover out of something like chicken wire to prevent the predators from gaining access to the water.