So you're going to laugh when you find out what it was.
Not a spy.
Not a Great Spy.
Not Peter having Optics, the +1 vision would not have been enough.
Not any kind of Guerilla II Archer from SB running onto a hill and then back.
It was this:
So for Sitting Bull to gain City Visibility on Canal City, he would need
1.2 * (100 + 365) * Modifiers
He had +13% distance penalty since Canal City is 8.5 tiles from his capital.
He had -20% trade route bonus
He had +2% cost modifer
No bonus for Confu. We are both running it.
1.2 * (465) = 558
Floor (558 * 1.13) = 630
Floor (630 * 0.8) = 504
Floor (504 * 1.02) = 514
Sitting Bull needed 514
against us to gain City Visibility on Canal City.
Sitting Bull had 540
against us because he has a Castle, and he put everything on us all game.
SB saw Peter's workboat once it got within 1 tile of Canal City.
If Sitting Bull had been able to do spy missions against us and spent some of that 540, then SB and Peter would not have met for a while.