I just got back from a short vacation. When I looked at my 5 gallon shrimp/fish fry tank, I realized that one of my apple snails (pomacea bridgeii) was missing. There are hiding places in the tank, so I didn't think anything of it. When I went back an hour or two later and still couldn't find the missing snail, I began to worry about him/her... that's when I noticed a snail shell sitting beside the tank on the floor (the tank itself sits on the floor). I picked it up and realized that the hatch was tightly shut. Seeing this as a good sign, I put the snail back in the tank, where it is now happily doing clean-up duty for me again. I've heard of snails doing this but have had this kind of snail for more than a year now and have never experienced them getting out of the tank. Because I just got back from vacation, I have no idea how long the snail had been out of the tank. It could've been only a couple hours or up to a week. Either way, I'm amazed he's still alive!