Oh wise Anonymous pundit, you get it by looking at the 12v battery which is doing the leeching -- the traction pack runs off the main battery, unless I'm mistaken.
When the vehicle is off, the 12v battery should be powering something about equivalent to a smartphone, so 4.5v sounds about correct. Amperage should be minimal until some mechanical process is kicked off, like unlocking the doors, turning on A/C, powering up the antenna(s) etc.
What this really feels like to me is that some secondary process controlled by the computer that's backed by the 12v battery isn't going into standby correctly -- likely culprit would be the wireless communication radio transmitter (not sure what type this car uses, so leaving vague).
One thing for owners experiencing this leakage to do would be to try parking their S in the garage overnight one night, and then outside the next night, and see if the consumption is visibly different. If it isn't, there's a design flaw to be tracked down. Has this system been EnergyStar rated?