When i worked at the dealership, I've seen a leaking steering box. the fluid was seeping through the cast, the seals were all fine, the metal was so poreous, the the truck was literally pissing out p/s fluid!
Post some pics! That would be interesting/helpful for others diagnosing leaks. I would've believed mine was doing that at one point, but it turned out to be the drain plug wasn't sealing well enough. A little thread sealant and it was a done deal.
Use to see this on aluminium rims during winter up here. Someone uses aluminiums rims not intended for use during winter and the salt corrodes the it so much that in about a month its starts leaking air and at the end of the winter you it leaks so bad you filling air a couple of times a week. I have heard about tranny pans doing the same thing but never seen it in person and dont run the ty during winter.