Originally Posted by NorthPacific
Between where and where B
I would first disconnect the cable from the battery
, just before the distribution panel and measure between this wire and the bolt/connector from which you remove the cable. A typical small VOM will measure Amps in line but beware most of them have a time limit (say 10 secs measurement every 5 minutes or so). If there is a loss, the VOM will read a value similar to the Trimetric (as you said about 0.5A).
There are also clamp meters that simply clamp around any wire or cable and tell you how much energy is flowing in the wire. You may be able to borrow one from a fellow buddy.
If you do have the loss, then I normally disconnect circuit by circuit until I find the iffy one. Remember a bad switch can leak too - leading to a small loss. This can be found by measuring their impedance in the off position.
When you disconnect the bad circuit, your Trimetric readout will switch to 'normal' (low) value. That's too how you know which circuit is 'bad'.
As someone above said - make sure there is nothing that actually draws the energy - say a gas solenoid, a small lamp, an inverter
in Standby, a control lamp ... We often forget such things.
Let us know how it goes.