Not generally a problem. It's called "misting". The outlet to air of the Rohloff is into the centre of a hollow axle and then to the outside. If you do not wipe the hub area for a few weeks, you will find a thin layer of oil on it.
Trivia: Your Rohloff isn't actually air- or water-tight. That would make it far too heavy. If you ride it through water over the hub, you should change the oil at the first opportunity.
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If what you're now experiencing worsens to a reasonably-sized puddle under the bike, then it is worth investigating. But, before you panic, put your finger in it and taste it to ensure it is oil and not water that was "misted out" of first. Even if it is oil, it is usually no problem, especially if you either overfilled the gearbox (1) or there was a large change in the ambient pressure (a change of weather, which you can see if you have a barometer, or of temperature at the change of seasons or just if your bike is inside and you switched the central heating on or off).
More Trivia: Enough oil clings to the Rohloff gearbox teeth and axles to lubricate it under all normal conditions, so the box can never be empty of oil. Everything that is misted off is either unwanted water or surplus oil.
Instances of Rohloff boxes being ruined where the symptom is a little visible external oil are nil. The normal symptom of a Rohloff box being broken is a graunching noise or, more common, that it just stops working.
Don't worry; it is very hard for a conscientious owner who performs the regular oil changes to break a Rohloff.
(1) This happens if you didn't drain the cleaning oil fully, or if you put in more than 15ml of all-seasons oil. But this isn't a harmful condition, just slightly messy.
