I just experienced this. I have a 20gal with a lid and I just changed filters from a waterfall type to an internal one as my tank is in my room and i couldnt stand the sound of the waterfall. I didnt think that opening was a big deal. Well 2 days later I couldnt find my snail. I thought maybe he was in the back hiding under the leaves where it gets dark (his favourite place). That night I was washing my floor and I noticed a shell on the ground near my power bar for the tank. Turns out he must have escaped. I felt so bad! Poor little guy.

So I recommend covering it up!

 

IME, Mystery Snails will not go far above the water line except to lay eggs. Nerites on the other hand, are escape artists and will frequently escape even where there is the smallest hole. I once found one a good two feet away from my tank on my kitchen counter.

 

looking for food????? lol

 

Agree with above. Only my female mystery snails have ever left the water to either lay eggs or scout for a future place to lay them. When they're laying eggs, they like to be completely out of the water. Leave at least 2 1/2" of space above the waterline for egg laying so they won't feel a need to go further up. They can also get eggbound if they don't have a place to lay them.