apple snails and mystery snails lay their eggs above the waterline so they will almost always leave the water given the chance either to lay eggs or to leave the area. Sometimes they like to explore for a nicer "pond or river" if they can find one. Of course you don't have a pond or river in your house so once they get dry they close up and wait for a rain. I found out a mystery snail can't last 6 months out of the tank though. I had one disappear and couldn't find where it went until I did some rearranging and moved an end table and found him under there. Usually you can follow their slime trail down the tank and across the floor or wherever they went, mine just happened to somehow make his trail disappear or else my cats found it first and cleaned it up before I found it. you should really have a closed lid with no holes they can fit through or as few as possible, this will limit the escapes. Kyle