I've only ever had one snail get out of a tank and I usually have gaps on my tanks. I really think just like most fish and other critters, the only time they want to get out of the tank is if the water parameters aren't something that they like or maybe not getting enough food.
Apple snails are usually sold as "mystery snails" in the lfs. Black, Gold, Green...same snail usually
They're a lot of fun, and I noticed around here that the green/blue types are becoming more popular. I picked up a green one a long time ago...then had my green and yellow one together for a couple weeks when I moved and then seperated them again and then the momma layed a clutch of eggs and I let them hatch out of pure curiosity. NEVER AGAIN. Even though it was kinda fun the first time around
Gave baby apple snails to everyone and their brother.