12V Battery Drain/Can't Start

a couple of mornings ago without any prior problems, my 2010 Prius was completely dead. I was able to get to the 12V battery and remove it , recharge to 12.5 V and reinstall it. The dash lit up brightly for a few seconds, the car would not start, and the dash finally faded. The power switch wouldn't turn the dash off, it eventually went dark. When everything seemed off I could hear a definite whine coming from the engine compartment, probably explaining the battery drain. I have again removed the battery to charge it and I'll try to get error codes. Anyone ever had a similar issue?

 

Thanks guys - it was the 12V Battery. I recharged it and reinstalled it but I guess it couldn't hold a charge. When I installed a new one it solved everything immediately.