4 more question:
1: You say the leek is from around the rear sprocket - there are two seals in this area. Do you think can you work out if the oil came from the seal that is between the hub and the sprocket or from the seal on the outboard side of the sprocket. I realise that is is hard to tell but take a good look - it makes a lot of difference. (This is actually the same as expr 2nd question, just reworded)
2: have you recently cycled through long grass (axle hight). One common thing that can create a leek is if vegetation has wrapped itself around one of the seals (most notably inboard of the sprocket)
3; Do you own the sprocket removal tool, chain whip and a shifting spanner?
4: Have you ever removed the sprocket and would you feel comfortable to do so?
Don't worry too much, it is perfectly safe to carry on riding a hub even if it has an oil leek - however yes this does need to be sorted.