On my '12 anyway the 12v battery can also charge while just sitting, not running or charging. On more than one occasion I've been in the garage when it's quiet and heard a humming. I look over and one of the charge lights are flashing(I think flashing). Anyway I looked it up once and it meant it was taking power from the traction battery to charge the 12v battery, cool
FWIW I've never heard my '13 doing this, although I could just be missing it......
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a