I had the rear wheel off, as normal, I washed it before doing work on it.
I pulled the brake hub and noticed a strip of rust on either side of the drum.

Yes, capt. obvious, it got wet.
But why the streak down the center which maybe a bit of grease.
You'd think that if the sides were wearing that it'd be the same all the way across.

I have no idea why the "no rust" zone....maybe you are correct that a bit of grease came out of the bearing and landed on the center and lubricated that area....it does appear the bearing was weeping a bit of grease out.

However.....the rust is a good example of why I never wash a bike unless I can take it out for a 10 minute ride before putting it away.  The water just lays in lots of places and corrodes the metal parts unless you take the bike out and use the brakes and let the wind and engine heat dry things out.  The cylinder head, brake drum, rear pulley hub, and all the wiring connectors can hold onto a lot of water without that lap around the block to get the water to go away.  It is better to put the bike away dirty...than it is to put it away wet.  With the bike on the kickstand the water tends to stay inside the brake drum.....rather than drain out.