lol ya i've had that happen even IF they're fake plants...if you've brought ANYTHING from another tank, you could bring snails...and that includes when you buy a new fish...theres always a chance that one could be on the net, or could have been on the ground when the lfs employee was chasing the fish into the ground (petsmart people do that a lot..drives me crazy they way they just torture the fish!!! but some of those people are good)
I've had that happen before, but luckily i saw the snail in w/the sand that was in the bag before i put the fish in the tank (i usually just dump the whole baggie full of water in the tank..after i acclimate of course)
also, have you brought in any used driftwood/rocks/gravel? if so, thats prolly your best bet of where it came from