Do not store the HV battery at full. Run it down to somewhere north of 50% say 60%ish. This isn't absolutely necessary but it will extend the life span of the HV battery. Then pop the hood and unhook the 12V battery. Keep the 12V on a float charger. If you don't have a float charger just put the 12V on a charger before hooking it back up to the car when you get back. If you don't have a 12V charger then the car should charge it back up next time you drive it, but that is hard on the 12V battery.
If you just leave it plugged in the car's electronics will eventually kill the 12V battery after a month or so. The car will not draw grid electric to keep the 12V charged. If you don't want to disconnect the 12V battery you could just hook a 12V float charger to the battery, but you run the risk of the car electronics drawing more power than the float charger can keep up with.
2014 Blue Candy FFE
http://www.fuelly.com/car/ford/focus/2014/triangles/303811 (since this forum doesn't allow BBcode in sigs)