Lady D is right, thought there is a variety that lays underwater as well, isnt there?
Perhaps it was on the back of an intake tube or the waterfall/output area on the filter? I know my snails climb that thing like there is no tomorrow.
As I said I've never had experience with them, but the website I pointed to said that a few may die off within the first few weeks?
It honestly sounds like you'll have more snails than you'll want to keep.

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Have fun with the little guys!