There's a small hole between the resevoir cap and the resevoir which allows the diaphragm to "breathe". If your brake had a badly done re-bleed, some oil could have been trapped between the diaphragm and the resevoir cap causing it to escape through that hole. In this case, it won't leak forever, only until there's no oil above the diaphragm. Surely you could just remove the cap and have a look inside? The cause will be pretty obvious once you dismantle the lever.