I don't have FW snails but my SW snails find ways out all the time , I find them in my sump and where ever they want to go.
in my overflow even on the outside glass , I just pluck them off and put them back in the tank , Snails are explorers they just go anywhere they want , they can go up to 2 weeks out of water as they have a water/mucus sack inside there shell to keep them moist , even if you let them wander on the outside glass they will find there way back into the tank as they can sense the water to re-hydrate , they wont wander to far away from the water, I just let mine do there thing , it's neat cleaning the outside glass seeing all the snail paths you can see where they come out and even go back in .
those stupid snails aren't as stupid as you think they are .