SF, The OP here posted a suggestion of video surveillance in a different thread in regards to catching the culprit for vanishing fish. To be honest, he came up with the idea himself - which isn't that horrible of an idea. People do it all the time to find out what's going on when they aren't home - often with YouTube worthy results. And it's easy enough to do with a cheap digital camera.
3 snails don't seem likely to be digging up plants, as was also suggested - and which OP is pretty adamant is not happening. I'd also blame the cories, as they (and the khuli loaches) are the culprits in my tank. OP wants answers, and surveillance will provide them. Bottom line, it was his own (uniquely inventive) idea to begin with.