i'm not buyin
into the theory that they were on the lace cat - you woulve have noticed a clump of eggs - unless their microscopic which i dont think they are - also i think a snail is gonna have to lay eggs soemwhere more stable than a moving fish - i can't picture your cat holding still to let a snail lay some eggs
i know someone will say well the cat coulve rubbed up against some eggs in the lfs tank but i dont see that as likely either
also maybe they came in with the mystery snails but if you said there were 50-60 of them that were in your filter then this means that in the course of 12 hrs there was a mass snail migration up the side of your tank and into your filter! lol seems unlikely to me - at least you or gulp or someone woulve noticed!
food is outta the question too they didnt come from there -
this is a tough one
maybe some mean kid snuck in in the middle of the night and put them in your tank !