Well did a test with the airpods and case. First charging both to 100%, then switching Bluetooth off on my iPhone and not using the airpods for 24 hours.
The case lost only 2% ! (airpods still at 100%)
That seems to strongly suggest the standby connection between iPhone and case (through the W1 chip) is the culprit for the high standby drain.
Not sure what to think of this as I really do not want to have to switch off Bluetooth when not in use (I also use Bluetooth connection with my car radio).
Probably will be reason enough for me to return the airpods. That or wait for firmware or ios update of some sort to sort out the standby drain...