While doing some electrical work myself, I was looking inside my toolbox, and found my clamp ammeter !
That is one thing that you could try too ?
That place you have disconnected , that has one heavy cable coming, and two leaving, to alternator & starter.You could test both those wires, easily safely & fast with a clamp ammeter.
The same clamp could be used at the battery side.One wire at a time.It shouldn't show much amps at all, when everything is turned off.
You put one wire at a time, inside the clamp claws.It should show/measure the current running through that wire.
Test those two wires going to alternator & starter.If you have significant reading on either one, follow that wire, to where ever it goes, to pinpoint the culprit?
I haven't used clamp ammeter myself, on a car, but it should work ?
On A measurement & DC side.
Or am I wrong ????