My ECU draws very little, sub 10mA just sat with the immobiliser LED flashing. Not enough to drain the battery as you describe. I did have to fix a very similar problem on one of my son’s cars recently. I traced it back to his fancy wireless. He refused to part with it so ended up fitting a separate power switch. If you have one and have no way of checking it I would suggest perhaps disconnecting it for a week or so.
