Where did they come from?

[ZombieFriedNuts]

There is four stacks of horded Sarmatian in my land but its wasn’t possible for them to have got in my territory without me knowing I have it sealed up. They just appeared out of nowhere is this a bug can anyone explain it. And no they don’t have any ships.
I’m playing BI campaign

[Quillan]

I've heard that happens sometimes, though it's never happened to me. What HAS happened (repeatedly) is that I've had the Danube river crossing blocked by forts, been attacked by a horde trying to cross the bridge, I win the fight, and the remnants retreat ACROSS the bridge, through the fort, and off into my territory!