I can’t find any description in the NC statements where they mention painting an SOS or needing to, as already mentioned earlier, the NC crew had already made radio contact regarding rescue.
Another possibility is that one or more of the missing NC sailors may have actually survived and been left behind (discussed in multiple previous post)
Also, the missing NC 4th lifeboat, also from previous post, and missing NC crew may have got close during the abandonment. Maybe they washed out and weren’t able to row back until everyone was already gone. Probably, that specific scenario was already discussed too.
Anyway, a castaway NC sailor may be more likely to paint an SOS there. At least more physically able and more aware of how to do it. Though Amelia who was not afraid of heights and if she got some strength, could do it too, maybe during cooler hours.
Another point. From looking at old and new pictures of the Norwich City, the upper part of the front bow was painted recently with white paint. From wreck pictures the superstructure forward of the front mast looks like it survived the fire, mostly. That may be where they kept the white paint, close to the white paint on the ship that needed periodic re painting.
It could even be a NC sailor castaway who painted the possible SOS was part of the NC crew who painted the upper bow white to begin with. Using a similar technique described in my original PDF attachment. In any event white paint could have survived on the ship.
Maybe the NC castaway eventually sailed off in the missing 4th lifeboat after running low on supplies. They would have had a fair amount of the left behind usable supplies to maintain their strength for a while to perform tasks like that. If they found their way back to the island and/or 1st camp