Many have doubted this theory that I have come up with until someone else witnessed this besides me.  There was a tank I was testing DIY CO2 in and had not changed a single plant in that tank for over a year.  However one day I noticed a small snail in the tank.  I had been watching the tank very closely for at least 8 months from when the last plant was put in and never saw snails or eggs.  So I was puzzled where this came from.  I knew I had some snails in my tank upstairs but never in this basement tank.

Well.. one day I was in the kitchen and I was walking through and heard a crunch.  I looked down expecting cereal but instead found a smashed snail.  Now keep in mind this is over 25-30 feet across carpeting and was about 10 feet across vinyl flooring.  I have also found them on the landing going towards the basement which is down more carpeting, and tile.  One could assume that a lucky snail instead of drying out (which I have found those as well) could successfully migrate from one snail infested tank into a non-snail containing tank.  Once a few friends make the trip you end up having rapid reproduction to the point where it becomes as infested as the other tank.