Are you sure they're dead? Some snails, and I'm not entirely sure if this applies to mystery snails, but I've seen many pond, ramshorn, nerite and assassin snails float to the top and continue grazing the glass once they've gotten close enough to attach themselves. They can fill their shell with air and float around for a bit. Usually with dead snails, they're at the bottom of the tank on the substrate laying motionless,usually indicating something is wrong such as illness or dying. That's why at my job, I only pick off snails on the glass or actively moving around when a customer asks for snails.