If the shell was flatter than a turbo and when it sucked into the shell it had a pointy "trap-door" to protect the foot of the snail it was a carnivorous snail. Remove it right away.
This snail (the sundial or heliacus) is often a hitchiker with button polyps. That is how I got all 3 of mine. I took mine back to the lfs and they fed the 3 snails to their puffer.
The best way to look for them is at night when the corals are closed up. Use a flashlight and scan in-between the polyp heads and look for that checker-board shell. However... before you destroy anything be sure and check the web for pictures or buy these two books: Eric Bornemans "Aquarium corals, selection, husbandry, history" and Julian Sprungs "Invertebrates, a quick reference guide". I have used Julian's book for those quick ID's for two years and have let alot of helpful critters grow and reproduce. Some critters when described will sound bad and you may be advised to remove them... it's good to have books with photo's to refer to.