Are you sure that there's nothing else wired into the cig lighter hot line? That's often used as a convenient 12V source for accessories like a stereo, electric fan controller, etc. Try pulling the fuse to see if anything else besides the cig lighter is affected.
Also....not sure what alternator you're using, but I've seen cases where they can be wired improperly so as to draw current even when the ignition is 'off'. It doesn't affect their operation while the engine's running, but they run the battery down quickly after the engine's shut down.
Good luck tracking down the culprit.