I've had fish totally disappear from my tanks a couple times. Couldn't find a body anywhere, in the tank or out. Just gone. One was a honey gourami and the other a balloon blue ram.
The other was a 20g long invert breeding tank with cherry shrimp, ramshorn snails, and pond snails. A few MTS had gotten in with plants too. The tank was going strong for over a year, and then one day I realized that I hadn't seen any snails in a while. Normally, they were all over the plants, decor, glass, in the filter, etc. I hadn't been paying a whole lot of attention to the tank for a couple weeks, just enough to drop some food in or do a quick WC but didn't really stop and look at it. There was enough movement from the shrimp that I guess I didn't think much of it, and there was enough plant cover that I guess I figured the snails were in there. When I thought about it, I realized how weird it was to not have snails on the glass. Then I looked hard and only found empty shells. The shrimp were doing great, and the nerite snails I introduced a couple months later are just fine. The only thing I can figure is the planaria at the eggs, thus stopping the expansion and renewal of the snail colony, but I'm still baffled about what happened to the adults.