If you disconnect the alternator and connect an amp meter to the two ends you should be able to see a current drain, possibly a varying one, there shouldn't be anything.
If its not that, do the same with the battery with everything turned off, take one lead off and connect to one meter lead and connect the other one to the battery, if its reading more than about 50ma something is draining it, pull the fuses out one at a a time until the drain current drops, check what that fuse is connected to.
Peter