Check the quick connector where the hose connects to the slave/release cylinder.
The hose pushes in and if it isn't connected all the way, it will leak.
Push the hose into the slave cylinder, if it goes in at all, it is likely the problem. Refill and keep an eye on it for leaks.
If the slave cylinder is indeed the problem, then the transmission will have to come out, as the slave cylinder bolts inside the bell housing.
I would suspect the hose/tube quick connector though.