Can you remember at what position the screw was at when you parked up?
Oil would only leak from it if it was at the bottom.
Could the oily residue be some old oil / flushing oil that remained on the hub or spokes when you drained the hub?

I can remember spilling a little oil onto the hub of a motorbike when I changed the rear bevel oil (moto guzzi with shaft drive) and for ages a little oil would get thrown onto the rim even though there was no leak.

Put a piece of tape, any sort, over the screw after cleaning around it with isopropyl or something similar.  That won't stop any oil leaking, but it'll make it obvious if that's where it's coming from.  I'm skeptical that enough oil would get past the grub screw to form a pool, but you won't know till you investigate further.
How old is the hub?  My oldest one started leaking after about 15 years, but it's very inconsistent, sometimes it was after a long ride, sometimes when I'd just been to the shops, sometimes not for weeks, I tried to be consistent in the way I parked it, but it didn't make any difference. I occasionally think about getting it fixed, but instead just follow the oil change method in the Living With a Rohloff booklet, basically give it a better flush and drain most of the oil back out,  it's been fine doing that for the last 6 years.
It's up to you, but I wouldn't bother doing another oil change till it's due (I'd also not be buying kits, it's a very expensive way to buy what you need) Rohloff themselves say if all the oil were to leak out there's still enough coating the parts to be fine till the next change.

Thanks

Put a piece of tape, any sort, over the screw after cleaning around it with isopropyl or something similar.  That won't stop any oil leaking, but it'll make it obvious if that's where it's coming from.  I'm skeptical that enough oil would get past the grub screw to form a pool, but you won't know till you investigate further.
How old is the hub?  My oldest one started leaking after about 15 years, but it's very inconsistent, sometimes it was after a long ride, sometimes when I'd just been to the shops, sometimes not for weeks, I tried to be consistent in the way I parked it, but it didn't make any difference. I occasionally think about getting it fixed, but instead just follow the oil change method in the Living With a Rohloff booklet, basically give it a better flush and drain most of the oil back out,  it's been fine doing that for the last 6 years.
It's up to you, but I wouldn't bother doing another oil change till it's due (I'd also not be buying kits, it's a very expensive way to buy what you need) Rohloff themselves say if all the oil were to leak out there's still enough coating the parts to be fine till the next change.

Sounds like a leak from the screw - a bit of a coincidence to suddenly get a leak from some other place.

What volume of oil did you put in?   The recommendation used to be 25ml but is now half that.   If you had put in 25ml you may be OK?

I am now using blue hylomar gasket sealant on the plug.  It is a non-setting sealant and seems to work well both as a sealer and to stop the plug undoing, and is easily found in UK.  Obviously clean the plug and hub threads first.

Obviously just me, but I can still not understand why drop bars are used when riding off road.  But then I have never tried.

I don't get it either, though I'm usually of the opinion that things I don't get are probably not aimed at me.  It isn't just the drop bars, the other common elements of the sorts of Gravel bikes I come across - steep seat tube, short chainstays, short headtube, fast steering - wouldn't suit me either, on or off road.  However, there's a long tradition of drop bars off road, it's always been the default for cyclocross and it's the norm in all the nostalgic Rough Stuff Fellowship photos.