25A fuse under the hood pulled, plus all fuses in car and still a drain? All outside lights off?
Too much amperage to measure? I guess you are using an analog meter with a limit of 1/2 amp or so. You can substitute a small 12 volt bulb instead of your meter, the bulb will glow indicating when the drain (or short) is present as you investigate the problem. It will go out or become very dim when you find and eliminate the drain.
When key is off power is supplied to the electric cooling fan at the rad (if you have one), to some of the fuses, the starter solonoid, the headlight relay and of course there is a connection over to the alternator.

When something is turned on (ie a drain)it would be normal to see the 12V as you described