Thank you for the condolences. With this weekend's nice weather, I thoroughly cleaned the pond and removed all of the leaves, but there is no sign of the koi. I am completely baffled by what type of critter could have caught all of the fish, including several average size shunbunkins and sarassas goldfish. The interior edges of the pond are intentionally lined with stacked rocks and small boulders, leaving lots of nooks and crannies for the fish to hide. But something cleaned them out....
I added several small comet feeders, just to put some life back into the pond.