A buddy of mine has a 98 M3 that he explains has left him stranded 4 times
in the past few weeks with a dead/weak battery.  Occurs intermittently,
after the car has sat for some number of hours (e.g. overnight).

Alternator checks out fine.  Battery checked out good under load, and
accepts a charge, but was a bit old, so he replaced it.  Same thing.
Connections are clean and tight.  He connected a meter with the ignition
off, doors closed, etc to see if there was any appreciable (unexplained)
current flow.  Nothing more than a few milliamps.

Good alt, good batt, good connections, no unusual drain.  I've seen starters
that quick working when they get hot, but his problem occurs after the car
has sat for some time.

We're both puzzled.  What are we missing?
TIA.
O.