i had this question in transmissions class hehe........the amount of ATF in the case AND torque converter is more than the case itself can hold........from the minute you shut off the engine fluid is leaking back from the torque converter back into the main case .....under normal usage this is a very smalll amount thats unnoticed bcoz the second you start the engine it goes rushing back into the the torque converter ........if its parked for extended time the fluid that leaks back into the main case, over fills the main case and leaks out through the vent..........thats why cars parked for years always have puddles underneath them .....then when you start it ...all the fluid goes rushing into the torque converter depriveing the main case ....making it not engage and ppl usually say "oh no the trans is bad" when most of the time if you top it off with fluid it will run fine even after many years........im betting if you track where its coming from .....its coming from the vent........and a possible solution to this is if your going to store it for extended time get a pan that has a drain plug and drain the main case when it goes into storage ....just dont forget to refill it.