I had what I believe to have been DOS on a batch of OO soap. It appeared as a light yellow, like the color you have. It did not spread, but it did darken to a more distinct orange.
I made them in shape molds not a log, and only one of the molds had the spot. It never spread to the entire soap.
I also had it happen on a batch of Crisco soap. Several of the shapes had this and they also did not spread though they did darken.
I have had a bar with sunflower oil do something that looked pretty similar, after about a month it yellowed, and carried on yellowing throughout the whole bar.
Does it smell? Mine smelt like fried food. I would think if it smells, or begins to smell its off, if it doesnt it is discolouration as mentioned.
How strange! The times I've had DOS it's been little spots that are more on the orange side than that yellow you have there. I've never seen that yellow shade with DOS. Hmmm.
This was one of the only batches that I added melted Shea Butter to after the batter began to emulsify. The idea was to have more shea butter as the superfat.
Just so you know, there is no need to add any butter at trace. The lye is still quite active at this point and it will take whatever it wants. There is no guarantee that your butter will end up as your superfat.