StrangeRover there's another test procedure, which is from the blackbox website:
BECM Sleep Test
This test will determine if the BECM goes to sleep properly or it will stay awake.
Sit in the vehicle in the driver seat with all the doors closed.
Wait until the interior light switches off.
Switch on the ignition.
Switch off the ignition.
Open the driver door and leave the door open for the reminder of the test.
Switch off the interior lights by pressing the button for 4 seconds. On the message centre you can observer the message "Lights off".
The message centre displays "Ignition key in" for approximately 2 minutes.
If the display goes blank after the two minutes, then the BECM is gone to sleep, if the display is still on, then the BECM did not go to sleep.
Aftermarket Stereo headunit, amplifier, Sat-Nav etc? Burnt/sticky relais?
Fuse circuit by fuse circuit measuring is the systematic search approach, which I know for identifying a "current thief".