I agree with Rual. I can't see what is wrong. Your Rohloff box has less oil on it than mine, and I know for a fact oil on mine is just breather oil, perfectly normal; I wipe it off every New Year's day. If your hub doesn't make graunching noises, I would assume that:
a) it is simply condensation from the change in seasons (I've had pools of water under my hub, eventually traced by elimination to the heating coming on and switching off again at night, but terrifying before I learned how one lives with a Rohloff), or,
b) that you didn't drain the hub properly before you put in the 15ml of oil you mention.
There's no harm in writing to Rohloff or, if you're nearer to them, SJS. Better to look a bit silly than to have to buy a another box.
However, if you bought a secondhand box I assume you're on a budget, so you must consider the possibility that, to cover their own backsides, they will call in your box and disassemble it to be absolutely certain that they can give it a clean bill of health, and charge you for the labour since you aren't the first owner.
One more consideration: Do you have service records for the box from the previous owner? If the box has been regularly serviced (every year or every 5000km, whichever comes first, or close enough), then you shouldn't worry unless the box is audibly destroying itself, a very unlikely scenario.
See, according to Rohloff, as you clearly know if you know to put in only 15ml of oil, the oil that adheres immovably to the gears is enough to safeguard the internals against mishap for at least a year in any kind of use. This makes the box almost indestructible if it is serviced once a year, which is why I question any "problem" that isn't accompanied by the noise of a gearbox destroying itself. Even if the seals are damaged, personally I would wait to see how long the loss of oil persists, at least to ascertain that you didn't overfill the box by not draining the cleaning oil thoroughly.
