Could have been MTS snails in the dirt they gave you or eggs that eventually hatched. I have taken a bag of gravel from a tank, rinsed with hot hot hot water, dried, put in a bag, let sit for 6 months, gave it to a roommate, he put it in his tank and snails came out with 10 minutes. They can live dormant for a while, even without water.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps