Battery drains due to under-hood light circuit?

Thanks for the info!
New data point: I stopped at a local garage and chatted with the mechanics about this. One senior guy said "See if the alternator pulley is magnetized".
I tried it, and yes! With battery connected, fuse installed, and the 3-wire plug attached to the alternator--key off--there's a definite magnetic attraction from the pulley. Disconnect any of these things, and it goes away.
At this point, really, how could this not be *some* problem with the alternator? My only other idea of a cause for the drain was some kind of soft short in the wiring, but that wouldn't cause the magnetization.
I'm *this close* to just ordering a rebuilt alternator and trying it.

Success at last! The replacement alternator (rebuilt by Remy) reduced the battery drain to a few milliamps.
I had to buy an air impact wrench to loosen the mounting bolts, and a soldering gun to connect the wires, but what I saved doing it myself paid for that and a lot more.
The hardest part was installing the new serpentine belt, but I was able to do it with the help of a friend.
Thank you for all your helpful advice!