The only thing I can think of is that the snail has it's tentacles retracted, I guess would be the word. When a shrimp gets too close to my snails, the snail knows the shrimp isn't going to do any damage to it, so it doesn't hide into it's shell, but sometimes it will shorten up and sort of retract it's tentacles. Somewhat hard to explain, but that's the only thing I can think of, check on it a bit later when it's climbing around and see if they are there.