Bit puzzled by all this (from an academic viewpoint 'cos I don't own a Mk4). Elsewhere in the forum, I think someone has pointed out that the handbook says you should drive the car for at least 30 minutes every 3 months in order to keep the HV battery happy. By implication, the 12v battery ought to be OK with that, otherwise surely Honda would specify something different? So if it drains quicker than that, isn't there a fault? And if you have to keep the car trickle-charging when not in use, are you not simply disguising that fault?
I've never trickle-charged my Mk3, and once left it for nearly 7 weeks; it started first time.