Alan, 12.56 is about normal for a battery of some age sitting for a week and should not be of much concern. As for the voltage issue, that is a bit more difficult to track down. it could be something in your battery monitoring system or an LED light or a slight short. The only way I know of to check is to open up your electrical
panel with the shore power
disconnected. Put the battery switch on both. Look for a connection that might be attached to the post for the switch. Then start disconnecting the wire from each piece of equipment
on the panel to see if the voltage goes away. If it does you have found your issue. Also disconnect the battery charger
from the batteries to see if that is the problem. Also look for things like bilge
pumps that are connected directly to the battery and not to the panel. Finally check with the shore power cord unplugged from the boat.