LOL Ive gone through that same thing before! When I first got my snail (pictured above) he was about the size of a nickel. I came home from work and didnt see him in the tank and realized he could have fit throught the opening in the back! I looked all over the place and couldnt find him and was so stressed out! Then I moved a plant in the tank and saw him sleeping peacefully between the leaves on it.
Awhile after that I was cleaning out that tank and had the lid off for a little while. I came back to check and make sure my fish hadnt jumped out (not thinking I needed to worry about the snail!) and there Snail was, out of the water, perched at the top of the tank about to climb down! I never knew how fast snails were till I got an apple snail.
I actually read a story about someone whose snail got out of the tank and was missing for about a month. The person figured it had died and kind of forgot about it. Then they moved and under the tank stand was the snail, all sealed up in its shell, still alive. So as long as they can stay closed they will retain moisture.
I am not recommending keeping your snail out of water, though!