Is this normal for pond snails? SOme snails usually climp above water level of the tank and sit there until it dies. It looks like the vaporize in the shell but the suction part that keep the sheel attached to the tank wall is still there and wet.
I have Nerite snails that regularly Leave the water overnight I have to do a check around the tank and under the hood every morning and return them to the water . I have no idea why they do this I can only assume that when the lights are out there is more condensation on the cover glass / hood giving them a damp surface to move on as they look for food It seems when they then hit a dry bit they clamp up to retain their water in the shell and if I dont find them within a day they dehydrate and die. I think in there natural environment they may well leave the water at night but the surrounding area to a pond /river would stay damp allowing them to return on their own. In a tank once they leave the water they find conditions to dry and are unable to return on their own . Just a theory