If fish disappear and nobody sees or hears anything it is usually a heron.
And they usually put your pond on their menu so it will come back.
I'll attach the heron hints if you haven't seen them before.

Heron tips:
- string fishing line around the pond
- put in an electric fido fence
- netting over the pond.
- a motion activated sprinkler
- a brick flue at the bottom of the pond
to give the fish a place to hide
- Migratory Bird Act forbids lethal methods,
heavy fines and jail time take away from pond time ;-)

good luck!

k30a