I have the same problem with my car, a 350 motor and th350 tranny. The car sat for along time in the previous owners driveway. When I was considering the car, it would not go into gear untill we added about two qts of fluid.
I bought the car and after it sat in my garage for a month or so, I noticed a large puddle of tranny fluid under it.
The weird thing is I know its not from the rear seal and I am not 100% positive it from the pan gasket. So now I am wondering if its from seals or vents too. I hope I don't have a leak in the case.