Snails lay eggs. The eggs get into drip water from infected (technically occupied) orchids. From there lots of thing spread. In my case (a Wardian Case) the darned things migrate.
OMGosh I had a bucket full of rain water that I poured on the hanging orchids baskets!!!!!!!!! Maybe that's how they got up there! But the water looked completely clean.
They are either very acrobatic or magicians or something but they do find their way up there and get out before you wake up. My Phal was on its side to avoid root rot in a wire basket, so he had to first blimb up the fence then slime his way through the wires to the plant.....not to mention what they were doing to all my other easy to get to plants. Im happy to report that the snails are all or most resting in peace now.