Bullfrogs

I imported a dozen big bullfrog tadpoles from a company. They all looked
fine -- second year tadpoles. Some had hind legs starting. #12 died. As
an experiment, I let #11 out of his protection screen box to see the koi
reaction. They looked him over but didn't pay much attention thereafter.
So, I assumed, it would be ok to let them all out. That was about four weeks
ago. Haven't seen any since. The pond is concrete, about 30 x 40 x 2 feet
and full of string algae, lilies, etc. One center area 6 x 6 is 3 ft deep.
The seven koi are big, well fed, about 25 years old. Any ideas anybody??
Do they hide until dark? Will someday I see some sitting on a lily pad? Do
I start over? Somewhere I read that koi don't like to eat tadpoles but they
are always rooting around in the algae for live food including goldfish
hatches. Could they have gobbled them by mistake? Thanks.....
Bruce Matthews
Monterey