I’ve had a similar issue with my battery getting drained by a dark current. I’ve traced it down to a failing alternator. Something inside the alternator fused/melted/do not really know but was messed up and as long as the battery was physically connected, it would drain. Once I replaced the alternator, the problem went away for a while. Just out of habit, I now always disconnect the battery cable at the battery.
Just FYI... the local auto parts store rebuilt alternators have been very low quality. Still wish I had my orginal one.
Hope this helps