I'm just curious how well you cleaned the BH?
I know that fluids can get trapped in the casting voids. Heating/case expansion can allow trapped fluids to leak.
For instance, I blew a coolant line. Replaced it and cleaned everything up. Few months later replaced a clutch and there was still coolant hiding in the voids. It did appear to leak every once in a while but nothing ever went low.
IIRC it's only the lowest void that has a drain hole. The rest and closed.
Just something to think about.
OH, you're not playing dumb ...