Applesnails, especially females will leave the water to deposit eggs,
If there is no cover don't be surprised to find them on the floor one morning,
all it takes is a small opening, Just had to do a couple repair jobs myself on a few of my snails,
the fall can crack them and they will die if left exposed for any length of time.
Even other snails will occasionally leave their tank, I see it all the time on my open topped tanks, they move between tanks, I'm constantly having to put them back in their correct tanks.