Caltoa:
That's bad, a power surge and PC destroyed.
You don't know what else is damaged.
I suggest to use a multimeter and check everything step by step.
I have done all the steps that I can think of :
disconnected each part of the system from the 12V supply when the reading was down to 9V, one at a time. In each case, there was no noticible change to the 9V reading.
At the end, I was left with a totally disconnected Mega 2560 ( all wires are on male header pins, so were physically removed ) and still no change.
So I suspected the Arduino and ethernet shield.
Removed the shield - no change.
Disconnected the Mega and connected a spare, brand new, Mega - same reading - so unlikely that the Mega was damaged and causing the drain.