I had something slightly similar happen, but assumed the snail (pond snail) came from the plants I purchased from someone...but that person has never had pond snails (just MTS's).
Another possibility that was mentioned was the gravel. Not sure how this could've happened, since the gravel was a brand new bag though...I suppose anything's possible.
Luckily for me, the pond snail never developed into pond snailS, despite the tank (with plants) having been up-and-running for more than 2 weeks.
25g Marineland Eclipse (30w T-8 ):
- java ferns, anacharis, corkscrew vals, giant hygros, marimo balls, java moss
- zebra danios (5), otos (2), others TBD
6g Marineland Eclipse (8w T-5):
- anacharis, asian ambulia, java moss
- white cloud mountain minnows (10 = 6-7 inches total)