Paul presented the following explanation :
My current car has vampire loads of less than 20mA.
The meter I used, can't give readings better than that
(that's the clamp-on ammeter reading). I would need
leads with banana plugs on the end, to be making a current
measurement without disturbing the electrical system.

With my car it is easy to test discharge whilst parked. There is a
copper link/busbar linking end of battery lead to the under bonnet fuse
panel, which has all the very large main fuses in it. I just connect mA
meter between battery +ve post and fuse end of bus bar and remove the
link. The measurement will not include any discharge via starter motor,
or alternator, but those are much less likely self discharge culprits.