I've always had preformed ponds and when it got close to hibernation time, I put a large plastic dishpan filled with river mud in the pond bottom and Speedy (male EPT) either used it or went under the dormant pump and hose that fed the watercourse . Saw him once during the winter when there was 8 inches of snow on the ground but the air temp was in the 50s. He soon went under again and I didn't see him again until mid-March and he went under again and finally came up for good a few weeks later. Sliders and Painted turts in the wild will most always bury themselves in the mud when scared off their basking spot and will bury themselves at night also. So mud in the bottom of large ponds (you didn't say how large yours was) is almost a necessity for aquatic turtles even if they are captive bred, if they are to be hibernated. Blood suckers also live in most natural pond bottoms so you would have to give your turts frequent exams for those parasites.
Also used a 1500 watt stock tank heater at the opposite end of the pond in relation to the hibernation area so the pond didn't freeze solid and kept a small pump (250GPH) going for water circulation as the filter was shut down. Fish don't eat nor do turts when the water temp drops to 50 so no fish or turt waste to foul the water. Plants die back but as long as the gases from decaying plant matter can escape to the air, that's no problem either. Black Trapdoor Snails are cold water snails and don't have a snorkel to breath air. They can live under the ice and will continue to munch on bottom debris all winter long and will help keep the algae in check. They eat the algae like you would mow your lawn if you were blind. I have 30 in my 265 gallon outdoor pond and will soon have more.
Keep 'em muddy and happy and I hope your lost ones are found.
George