The slightly over rated term that the hub sits in a "bath" of oil comes to be realised when its more of a small spill really!
Dave: I think the phrase you're looking for is "a permanent film of oil on the gears". Apparently that's about 12ml, and semi-publicly Rohloff says 15ml, and supplies 25ml just to make sure. I don't think it worth trying to save less than a tenner by trying to make a refill kit go twice, especially not on such an expensive piece of kit.
I had a small spill under my gearbox at one stage, not too long after an oilchange, described in another thread here, and eventually by draining and refilling the gearbox with the same oil that came out, determined that there was still around 10ml of free oil in the gearbox. I've since stopped worrying about whether there is oil in my Rohloff.
Stuart: I don't know if oil is hygroscopic, but condensation, when you bring a cold bike into a warm house or garage, is a mechanism for leeching water from air.
Rohloff calls the oil loss through the interference fit joints "misting" and I've seen it in the summer as a thin layer of oil near the joints, more so on the non-drive side for some reason. The manual, or a support document, does warn against overfilling.
André Jute
What I do when I can't ride my bike:
http://coolmainpress.com/iditarod1.html