So after some head scratching for a while it seems this maybe the culprit of some vampiric battery drain, around 300mA. Was hard as with a small draw it only really rears it's head if the car sits for a week or more.
Finally got bothered to pull out the multi meter and test it, and looks like despite seeming to be shut down it either still pulls current or activates the CANBUS and the ECU or something else pulls current.
Interestingly there seems to be a bunch of systems that do a staged shut down.
Car off (not even acc) with drivers door open pulls about 3.5A
Close the doors and lock drops to about 2.4A for a while
Then shifts to 1.9A, 1A for quite a while, down to 0.3A briefly before settling on 0.02A (20mA)
With the OBD device plugged in it never seemed to drop under 300mA like it was keeping something alive. I have another OBD bluetooth/wifi unit so might test that too one day and see if it does the same.
Will see how it goes though, over the next few weeks, but unplugging the OBD if I'm not going to drive for a bit isn't a hard thing to do.
