Mystery grease source - Alfine 11, Gates Belt drive, Shimano bottom bracket
Oil leaking from the hub has been an ongoing problem for me too.
When the bike was still new there was an oil leak from the gear changer side which went to Shimano (twice) before the internal seal was replaced (the external seal was replaced the first time to no effect.

Over a year later the leak started again and this time Shimano have had the wheel for almost a month and I'm starting to get frustrated. The decent thing to do would be to replace the hub and apologise for all the time wasted.



(Móż: edited in from comment because it seems useful, thanks OverTheRainbow)

On their "how we maintain your Shimano hub" page RideYourBike.com says:


  It is our experience that Alfine 11 speed hubs seep oil out of the main seal. 
  we now consider this normal! simply wipe off any excess and add more oil periodically.
  Newer hubs seem to leak less than previous. 
  If the seal is over-torqued it will deform and leak.