In message , Bob Minchin
writes
Nowhere in the question does the op say
"If he has disconnected te battery entirely and put a meter in the
circuit and no current is flowing, then no current is flowing and the
battery cannot be discharging *via any external means*. "

Just to be clear, if I park the car for a couple of days, with the
battery connected, the battery goes almost flat, but left 4 1/2 days
with the battery disconnected (earth lead removed from battery
terminal), the battery remains fully charged.

This morning, battery fully charged, I put an ammeter between the
battery terminal and earth lead. No movement. Repeating, with a door
open and courtesy light therefore in circuit, then I can see the ammeter
needle move. Not much, but enough to know it is working. Ammeter is a
standard car 2 inch circular 30-0-30.

Repeated but with a small 12v MES bulb between the battery terminal and
earth lead. No light until door opened.

--
Graeme